960-14Special

Publication

960-14

WWVB Radio 

Controlled Clocks: 

Recommended Practices 

for Manufacturers 

and Consumers

(2009 Edition)

Michael A. Lombardi, Andrew N. Novick, 

John P. Lowe, Matthew J. Deutch, 

Glenn K. Nelson, Douglas S. Sutton, 

William C. Yates, and D. Wayne Hanson

i

NIST Recommended Practice Guide

Special Publication 960-14

WWVB Radio 

Controlled Clocks: 

Recommended Practices 

for Manufacturers 

and Consumers

(2009 Edition)

Michael A. Lombardi, Andrew N. Novick, 

John P. Lowe, Matthew J. Deutch, 

Glenn K. Nelson, Douglas S. Sutton, 

and William C. Yates

NIST Physics Laboratory

D. Wayne Hanson

Time Signal Engineering

August 2009

U.S. Department of Commerce

Gary Locke, Secretary

National Institute of Standards and Technology

Patrick D. Gallagher, Deputy Director
ii

♦ WWVB Radio Controlled Clocks

Certain commercial entities, equipment, or materials may be identified in 

this document in order to describe an experimental procedure or concept 

adequately. Such identification is not intended to imply recommendation or 

endorsement by the National Institute of Standards and Technology, nor is it 

intended to imply that the entities, materials, or equipment are necessarily the 

best available for the purpose.

National Institute of Standards and Technology 

Special Publication 960-14

Natl. Inst. Stand. Technol.

Spec. Publ. 960-14

64 pages (August 2009)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 2009

For sale by the Superintendent of Documents

U.S. Government Printing Office

Internet: bookstore.gpo.gov Phone: (202) 512–1800 Fax: (202) 512–2250

Mail: Stop SSOP, Washington, DC 20402-0001
iii

FOREWORD

Radio controlled clocks represent a true revolution in timekeeping.

Clocks that synchronize to NIST radio station WWVB now number in 

the millions in the United States, and new sales records are being established 

every year. As a result, many of us are now accustomed to having clocks 

in our homes, offices, and on our wrists that always display the correct 

time and that never require adjustment. This NIST Recommended 

Practice Guide was written to provide guidance to both manufacturers 

and consumers of radio controlled clocks. Through voluntary compliance 

with the recommended practices listed here, manufacturers can benefit 

by continuing to develop more reliable and usable radio controlled products, 

increasing both consumer confidence and sales. Consumers can benefit 

by using this guide to help them select and purchase radio controlled 

clock products, to learn how the products work, and to help troubleshoot 

reception problems.

Foreword ♦
iv

♦ WWVB Radio Controlled Clocks
v

Acknowledgments ♦

Acknowledgments

This booklet is a revised version of the original booklet published in January 

2005. Although not a major revision, it updates information about radio station 

WWVB and the United States daylight saving time rules that were changed 

in 2007. It also makes a few small corrections to the time zone tables and 

maps. In addition, the recommended practices for radio controlled clock 

manufacturers have been revised slightly to incorporate new information 

collected by the authors since the original booklet was published. 

The authors gratefully acknowledge those who reviewed this document and 

made numerous helpful suggestions including: Michael Chiu and HansJoachim Sailer of C-Max Technology; Etsuro Nakajima and Aihara Fumikazu 

of the Casio Corporation; Glenn Burdett of the Spectracom Corporation; Rod 

Mack of Ultralink; John Rowland and Gene Fornario, the founder and comoderator respectively of the Casio Waveceptor forum on Yahoo.com; Tom 

O’Brian, Chief of the NIST Time and Frequency Division; and David Smith, 

Chairman of the Editorial Review Board at NIST’s Boulder laboratories. We 

also thank all those who have called and e-mailed NIST in recent years and 

shared their personal experiences with radio controlled clock (RCC) products. 

Their questions and comments inspired us to create this guide.
vi

♦ WWVB Radio Controlled Clocks
vii

Table of Contents ♦

Table of Contents

List of Figures.................................................................................................... x

List of Tables..................................................................................................... xi

1. Introduction .............................................................................................. 1

2. Technical Description of WWVB ............................................................... 3

3. Recommended Practices for Clock Accuracy, 

 Clock Display, and Controls........................................................................ 5

 3.A. Analog Clock Displays...................................................................... 5

 3.B. Digital Clock Displays....................................................................... 6

 3.C. RCC Controls .................................................................................... 7

 3.D. Compatibility with Other Stations..................................................... 7

 3.E. Signal Quality Indicator..................................................................... 8

 3.F. Antenna Orientation .......................................................................... 9

4. Recommended Practices for Clock Synchronization .................................11

 4.A. Initial Synchronization When Clock 

 Is First Turned On or Reset...............................................................11

 4.A.1. Analog Clocks (Hand Alignment).......................................11

 4.A.2. Digital Clocks......................................................................11

 4.B. Synchronization by Radio at Assigned Times..................................11

 4.B.1. Amount of Time Allotted to a 

 Synchronization Attempt.................................................... 14

 4.B.2. Status of Display During a 

 Synchronization Attempt.................................................... 14
viii

♦ WWVB Radio Controlled Clocks

 4.C. Synchronization by Radio at a 

 Time Selected by the User............................................................... 14

 4.D. Manual Clock Synchronization ....................................................... 14

 4.E. Synchronization Indicator................................................................ 15

 4.E.1. Digital Clock Synchronization Indicator............................ 16

 4.E.2. Analog Clock Synchronization Indicator........................... 16

 4.F. Adjustment of Display to Compensate 

 for Delays Introduced During Synchronization............................... 17

5. Recommended Practices for 

 Time Zone Settings.................................................................................... 19

 5.A. Time Zone Selection........................................................................ 19

6. Recommended Practices for 

 Daylight Saving Time (DST) .................................................................... 27

 6.A. Handling of Transition Days............................................................ 27

 6.B. Disabling/Enabling DST Switch ..................................................... 28

 6.C. DST Indicator .................................................................................. 28

7. Recommended Practices for Leap Seconds, 

 Leap Years, and the Two-Digit Year Code ................................................ 29

 7.A. Handling of Leap Seconds............................................................... 29

 7.B. Handling of Leap Years................................................................... 29

 7.C. Handling of Two-Digit Year Code................................................... 30

8. Recommended Practices for 

Hardware Specifications............................................................................ 31

8.A. Receiver Specifications.................................................................... 31
ix

 8.B. Antenna Considerations................................................................... 31

8.C. Local Oscillator Specifications........................................................ 32

 8.D. Battery Powered RCCs.................................................................... 32

 9. Recommended Practices for 

 Product Documentation ........................................................................... 33

 9.A. Mention of NIST........................................................................... 33

 9.B. Use of “Atomic Clock” Nomenclature ......................................... 34

10. Compliance Checklist.............................................................................. 35

11. Recommended Practices for 

 Consumers of WWVB RCCs .................................................................. 41

 11.A. How a WWVB RCC Works........................................................ 41

 11.B. Time Zone Settings...................................................................... 42

 11.C. Coverage Area of the WWVB Signal.......................................... 43

 11.D. General Troubleshooting Tips for WWVB RCCs....................... 44

 11.D.1. General Troubleshooting Tips for RCCs 

 That Won’t Synchronize at All..................................... 45

 11.D.2. General Troubleshooting Tips for RCCs 

 Off by One Hour or More ............................................ 48

 11.D.3. General Troubleshooting Tips for RCCs 

 Off by a Few Minutes or Seconds................................ 48

 11.D.4. General Troubleshooting Tips Concerning 

 Daylight Saving Time (DST)....................................... 49

12. References................................................................................................ 51

Table of Contents ♦
x

♦ WWVB Radio Controlled Clocks

List of Figures

Figure 1. WWVB time code format ...................................................................... 3

Figure 2. Analog clock display with digital inset for date information................. 5

Figure 3. Digital clock display with date information........................................... 6

Figure 4. Digital clock display that includes year information ............................. 6

Figure 5. A three-segment display indicating four levels of signal quality........... 8

Figure 6. Use of a broadcast tower icon as a signal quality meter........................ 8

Figure 7. RCC display indicating the date and time of the 

last synchronization.............................................................................. 15

Figure 8. World time zone map ........................................................................... 25

Figure 9. United States time zone map................................................................ 26
xi

List of Tables

Table 1. Radio Time Signal Stations Used for RCC Synchronization .................. 7

Table 2. Dark Path Hours and Duration for WWVB Signal at 

Selected Cities (Local Time).................................................................. 12

Table 3. Necessary Time Zone Options for WWVB RCC Products................... 20

Table 4. Recommended Time Zone Options for WWVB RCC 

Products Sold Internationally................................................................. 21

Table 5. Difference between UTC and Local Time for the Four Major 

Time Zones in the CONUS.................................................................... 43

Table 6. Potential Sources of Interference for WWVB RCCs............................. 46

Table of Contents ♦
xii

♦ WWVB Radio Controlled Clocks
1

Introduction ♦

1. INTRODUCTION

Through its Time and Frequency Division located in Boulder, Colorado, the 

National Institute of Standards and Technology (NIST) maintains official time 

and frequency standards for the United States of America and distributes 

these standards to the American public. Radio station WWVB, located near 

Fort Collins, Colorado, is one of the most important distribution sources for 

these standards. The station continuously broadcasts a 70 kW signal at a 

frequency of 60 kHz that covers the Continental United States (CONUS), 

and also reaches Alaska and Hawaii during the nighttime hours.

The signals from WWVB can serve as a convenient reference standard 

for time interval and frequency, but their primary function is the time-of-day 

synchronization of radio controlled clocks (RCCs). These clocks are now 

sold through a variety of retail channels to United States consumers, who 

install them in their homes and offices and rely on them as reliable and official 

sources of time, accurate to within 1 second (s) or less. These clocks are 

sold in a variety of forms, as wall clocks, desk clocks, wristwatches, or clocks 

embedded into a variety of consumer electronic products, including kitchen 

appliances such as coffee makers and microwave ovens, home entertainment 

equipment, and computer systems. 

As with all consumer electronic products, the quality of WWVB RCCs 

produced by different manufacturers varies widely. While many of the existing 

products are well designed and extremely reliable, some models cannot always 

decode the time signal even under the most favorable signal conditions, and 

some lack key features that limit their usefulness from a human engineering 

standpoint. NIST has produced this recommended practice guide for the 

benefit of manufacturers and consumers of WWVB radio controlled clocks. 

It recommends key features to manufacturers that their products should 

include, as well as key specifications that their products should meet. 

These recommended practices are voluntary. No manufacturer or consumer 

is required by law to comply with them. However, it is hoped that voluntary 

compliance with these recommended practices will lead to the development 

of more reliable and usable clocks, increase consumer confidence in 

WWVB RCCs, and, at the same time, increase the size of the commercial 

RCC marketplace in the United States. It is also hoped that this guide will be 

useful to consumers of WWVB RCCs by providing information that helps 

them select RCC products and troubleshoot RCC reception problems. 

The following sections contain a brief technical description of WWVB 

(Section 2) and product recommendations in seven different categories, 

including: clock displays (Section 3), synchronization (Section 4), 
♦ WWVB Radio Controlled Clocks

2

time zone settings (Section 5), handling of daylight saving time (Section 6), 

handling of miscellaneous time code issues (Section 7), hardware specifications 

(Section 8), and product documentation (Section 9). Based on consumer 

feedback received at NIST, the seven categories were identified as being 

important to the reliability, usability, and marketability of WWVB RCC 

products. Not all categories share equal importance, so a summary checklist 

is provided in Section 10 that identifies which categories are recommended 

as necessary for all designs and which categories are optional. However, 

we recommend that manufacturers comply with as many of these optional 

categories as possible in an effort to produce RCC products of the 

highest quality.

Many consumers will find the information in Sections 2 to 10 interesting, 

but some will probably want to skip directly ahead to Section 11, which is 

included for their benefit. It provides general information about WWVB 

RCCs, and it provides some troubleshooting tips for consumers whose clocks 

are not working properly.

Please note that it was necessary to write this guide in a general fashion. 

We identify for manufacturers the desired functions of WWVB RCC products, 

but a discussion of the technical implementation of these functions is beyond 

the scope of this guide.
3

2. TECHNICAL DESCRIPTION OF WWVB

The WWVB time code includes 60 bits of information, transmitted at 1 bit 

per second. A full minute (60 s) is required to send a complete time code 

frame (Figure 1). An on-time marker (OTM) is sent every second by reducing 

the power of the 60 kHz carrier frequency by 17 dB at a time that coincides 

with the arrival of the Coordinated Universal Time (UTC) second. Bits are 

identified by the length of time that the carrier power is held low. A 0 bit is 

sent by holding the power low for 200 ms, a 1 bit is sent by holding the power 

low for 500 ms. Frame markers are sent every 10 s by holding the power low 

for 800 ms.

Figure 1. WWVB time code format.

A second time scale is located at the radio station site in Fort Collins, Colorado. 

The two time scales, both consisting of groups of atomic oscillators, are 

continuously compared to each other and kept in very close agreement. The 

time difference between them is nearly always less than 20 nanoseconds.

Technical Description of WWVB ♦
♦ WWVB Radio Controlled Clocks

4

A complete technical description of the WWVB and its broadcast format 

is not provided here. A general technical description can be found in NIST 

Special Publication 432,

[1] and a more detailed description of the station 

operation, format, and broadcast control is available in NIST Special 

Publication 250-67.

[2] Both publications are available for download from 

the NIST Time and Frequency Division web site at http://tf.nist.gov
5

3. RECOMMENDED PRACTICES FOR CLOCK 

 ACCURACY, CLOCK DISPLAY, AND CONTROLS

All RCC products should display time accurate to at least within ±0.5 s, 

so that when the time is rounded to the nearest second, the seconds’ value 

is always correct. Tighter synchronization (to within ±0.2 s) is desirable. 

This prevents the human eye from detecting any errors when checking a 

RCC display against another independent time reference, whereas a 0.5 s 

error could be noticeable.

The chief benefit and a key selling point of a RCC is its time accuracy. 

Therefore, we recommend that all clocks display seconds, or have the 

option of displaying seconds. With some digital clocks, such as clock radios, 

the seconds’ digits can be made smaller than the hour and minute digits, or 

the consumer should be given the option to disable the seconds’ display if they 

find it distracting. Analog clocks require either a second hand or a separate 

digital display that shows seconds. A digital seconds’ display that can be 

turned on and off is often a good option because second hands are not always 

desirable on analog clocks. For example, a second hand on an analog alarm 

clock might be noisy enough to bother consumers who are trying to sleep. 

3.A.Analog Clock Displays

Analog clock displays should include an hour, minute, and second hand 

(or a digital display of seconds), and some marking or label indicating 

that the clock is radio controlled. The date and other information can be 

displayed (if desired) in a digital inset, as shown in Figure 2.

Figure 2. Analog clock display with digital inset for date information.

Clock Accuracy, Clock Display, and Controls ♦
♦ WWVB Radio Controlled Clocks

6

3.B. Digital Clock Displays

Digital clock displays should display the hour, minute, second, and 

some marking or label indicating that the clock is radio controlled. The 

digital display makes it convenient to also display the month, day, year, 

and weekday (if desired). It is also recommended that “AM” or “PM” 

is displayed if the clock is set to a 12-hour format, instead of a 24-hour 

format. Some digital clock displays use an icon of a satellite dish as 

a synchronization indicator (Section 4.E), as a signal quality indicator 

(Section 3.E), or simply to indicate that the clock is radio controlled. 

To avoid confusing consumers, we recommend that a picture of a 

satellite dish not be used, since WWVB and the other low-frequency 

(LF) time-signal stations (Section 3.D) all broadcast from ground-based 

transmitters and not from satellites. Sample digital clock displays are 

shown in Figures 3 and 4. 

Figure 4. Digital clock display that includes year information.

Figure 3. Digital clock display with date information.
7

3.C.RCC Controls

RCC controls should be clearly labeled and situated in such a way that 

they cannot be accidentally activated; for example, everyday handling 

of the clock should not result in the change of a time zone setting. 

This is particularly important in the case of wristwatches.

3.D.Compatibility with Other Stations

Some WWVB RCC products are capable of receiving time signals 

broadcast by other time signal stations located in other countries. 

This allows the clocks to be sold and used internationally. The time 

signals broadcast by other countries (Table 1) use carrier frequencies 

different from that of WWVB in some cases, and different time 

code formats in all cases, but the modulation schemes are similar. 

We recommend that products capable of receiving more than one 

time signal have a way of clearly indicating to the consumer which time 

signal is currently being received. We also recommend that RCCs are 

designed to change stations automatically (if necessary) when the time 

zone setting is changed (Section 5.A). For example, if a person wearing 

a wristwatch with multi-station capability travels from the United States 

Clock Accuracy, Clock Display, and Controls ♦

Table 1: Radio Time Signal Stations Used for RCC Synchronization

Station Carrier

Call Sign Country Controlling Agency Frequency

WWVB United National Institute of Standards 60 kHz

 States and Technology (NIST)

BPC China National Time Service Center (NTSC), 68.5 kHz

 Chinese Academy of Sciences

DCF77 Germany Physikalisch-Technische 77.5 kHz

 Bundesanstalt (PTB)

HBG Switzerland Swiss Federal Office of Metrology 75 kHz

 and Accreditation (METAS)

JJY Japan National Institute of Information and 40 kHz,

 Communications Technology (NICT) 60 kHz

MSF United National Physical Laboratory 60 kHz

 Kingdom (NPL)
♦ WWVB Radio Controlled Clocks

8

Figure 5. A three-segment display indicating four levels of signal quality.

Figure 6. Use of a broadcast tower icon as a signal quality meter.

Strong

Weak

No Reception

Receiving

to Germany, the wristwatch should automatically switch from WWVB to 

DCF77 when the German time zone is selected. In areas covered by more 

than one station, such as Japan, the RCC should automatically select the 

station with the highest signal quality.

3.E. Signal Quality Indicator

Inclusion of a real-time signal quality indicator is recommended so that 

the consumer can find the best location and antenna orientation for 

their RCC product while forcing the product to attempt synchronization 

(Section 4.C). Since the actual signal strength is not easy to detect due 

to other RF noise at 60 kHz, the signal quality indicator can show the 

“bit strength,” or current readability level of the signal; or it can indicate 

the progress of the decoding process in the software.

The signal quality indicator can be simple. A three-segment display 

indicating a low-, medium-, and high-quality signal is generally adequate 

(Figures 5 and 6). When the clock is not attempting to synchronize and 
9

the radio receiver is turned off or disabled, we recommend that the signal 

quality indicator also be disabled or removed from the display. Otherwise, 

consumers might mistakenly assume that the RCC is displaying the 

current signal quality.

3.F. Antenna Orientation

Most RCC antennas are directional and achieve maximum gain when they 

are positioned broadside to the transmit antenna in Fort Collins, Colorado. 

We recommend that an arrow or pointer marker is included on the RCC 

case to illustrate the antenna orientation. When used in conjunction with 

the signal quality indicator (Section 3.E), this type of marking can assist 

consumers in orienting the RCC to obtain maximum signal strength.

Clock Accuracy, Clock Display, and Controls ♦
♦ WWVB Radio Controlled Clocks

10
11

4. RECOMMENDED PRACTICES FOR CLOCK 

 SYNCHRONIZATION

This section recommends practices for clock synchronization. It is divided 

into four categories, initial synchronization (when the clock is first turned on), 

synchronization by radio at assigned times, synchronization by radio at a time 

selected by the consumer, and manual synchronization without radio control.

4.A.Initial Synchronization When Clock Is First Turned On 

or Reset

When a RCC is first turned on, it will begin looking for a signal and 

attempt to synchronize. We recommend that RCCs be designed to 

continuously try to synchronize on this first attempt until either the 

synchronization is successful or until the consumer decides to attempt 

manual synchronization (Section 4.D). We also recommend that RCCs 

that have not yet been able to synchronize should not run or attempt to 

display the time since their displays will be incorrect. Suggestions for 

what a RCC should display prior to synchronization are listed below.

All RCCs should be designed to synchronize without any interaction 

from the user. It should not be necessary for the user to set the display 

of the clock (move the hands, etc.) in order for the clock to synchronize. 

We recommend that at least two consecutive time codes are decoded and 

compared before determining that an initial synchronization attempt is 

successful.

4.A.1.Analog Clocks (Hand Alignment)

Analog clocks should align themselves with all hands pointed upward 

(pointed to the “12”) until synchronization is successful. The hands 

should remain motionless during synchronization.

4.A.2.Digital Clocks

Digital clocks should display 12:00:00 as the time prior to synchronization,

or flash the time display on and off, or display dashes instead of the 

hours, minutes, and seconds. The display should not increment during 

synchronization.

4.B.Synchronization by Radio at Assigned Times

To meet the accuracy requirements listed in Section 3, and to periodically 

Clock Synchronization ♦
♦ WWVB Radio Controlled Clocks

12

check the time code for notifications of daylight saving time, leap seconds, 

and other time code changes, all WWVB RCCs should attempt to 

synchronize at least once every 24 hours, and more frequently if possible.

If only one synchronization attempt is made, it should be made at night 

when the signal is the strongest. The signal is generally easiest to receive 

when it is dark at both the transmitter site in Fort Collins, Colorado, and 

at the site where the RCC is located, so the highest probability of a 

successful synchronization is during these hours. Table 2 shows the dark 

path hours (DPH) and dark path duration (DPD) in hours and minutes 

for six cities on the approximate longest and shortest days of 2004 

(June 21 and December 21), based on when sunrise and sunset occurs 

in those cities with respect to when sunrise and sunset occur in Fort 

Collins. The DPH are based on the local time in the selected city. The six 

cities were chosen to represent the northwest, southwest, northeast, and 

southeast corners of the CONUS, as well as Alaska and Hawaii. 

The information in Table 2 shows that the “window of opportunity” for 

synchronization ranges from about 4 hours (Anchorage summer) to about 

14 hours (Seattle winter). Attempting synchronization on the hour at 

midnight, 1 a.m., and 2 a.m. guarantees a dark path at all United States 

Table 2: Dark Path Hours and Duration for WWVB Signal at Selected Cities 

(Local Time)

June 21, 2004 December 21, 2004

City DPH DPD DPH DPD

Seattle, Washington 9:11 p.m. to 7:19 4:20 p.m. to 14:01

4:30 a.m. 6:21 a.m.

San Diego, California 8:00 p.m. to 8:30 4:46 p.m. to 13:35

 4:30 a.m. 6:21 a.m.

Caribou, Maine 10:34 p.m. to 6:04 6:36 p.m. to 12:38

 4:38 a.m. 7:14 a.m.

Miami, Florida 10:34 p.m. to 7:56 6:36 p.m. to 12:27

 6:30 a.m. 7:03 a.m.

Anchorage, Alaska 11:42 p.m. to 3:48 3:41 p.m. to 13:40

 3:30 a.m. 5:21 a.m.

Honolulu, Hawaii 7:16 p.m. to 7:14 5:55 p.m. to 10:26

 2:30 a.m. 4:21 a.m.
13

locations. Therefore, if only one synchronization attempt is made, we 

recommend that the scheduled synchronization time is set to be midnight, 

1 a.m., or 2 a.m. (local receiver time).

Manufacturers are cautioned to carefully choose a radio synchronization 

time and method that does not cause the RCC to display the wrong time 

(even briefly) on transition days to and from daylight saving time, when 

the time display should be adjusted forwards or backwards by one hour 

at exactly 2 a.m. local time (Section 6.A). 

If the receiver and signal processing firmware (Section 8.A) inside 

the RCC are of sufficient quality, it should be possible for successful 

synchronizations to be routinely made throughout the CONUS during 

both the daytime and nighttime hours. Therefore, we recommend that 

manufacturers design products that attempt synchronization more than 

once per day. This has the disadvantage of perhaps reducing the battery 

life on battery-powered devices, but it has the advantage of relaxing 

the quartz oscillator accuracy requirements (Section 8.C) by shortening 

the interval between synchronizations. For example, if the RCC is 

programmed to synchronize at both midnight and 4 a.m., it reduces 

the amount of time that the clock free runs on its local oscillator from 

24 to 20 hours, relaxing the oscillator specification by roughly 16%.

If the quartz oscillator is of sufficient quality to maintain accurate time 

for 24 hours (Section 8.3), the manufacturer may elect to offer several 

scheduled synchronization times, but to skip the times remaining on 

the schedule once a successful synchronization has been made. For 

example, a RCC could attempt to synchronize at midnight, 2 a.m., 4 a.m., 

and 6 a.m. If the synchronization at midnight is successful, it can skip 

the attempts at 2 a.m., 4 a.m., and 6 a.m., and wait until the following 

midnight before trying again. However, if the midnight attempt fails, the 

RCC will try again at 2 a.m. instead of waiting for 24 hours.

Since the small antennas used by WWVB RCCs tend to be very 

directional, it is difficult for a RCC to synchronize while it is moving. 

Therefore, it probably won’t be advantageous for wristwatches to 

attempt to synchronize during the daytime hours when the watch is 

being worn and is probably in motion. As a result, we recommend that 

wristwatches attempt their multiple synchronizations at times when 

the consumer is most likely to be asleep, and the watch is motionless. 

Several synchronization attempts during the night are recommended to 

allow for the varying “bed times” of consumers.

Clock Synchronization ♦
♦ WWVB Radio Controlled Clocks

14

Manufacturers should be aware of potential problems introduced 

by consumers who work nights or who never remove their watches. 

To meet the needs of these consumers, manufacturers can elect to 

allow an additional synchronization time (in addition to those built-in 

to the product) to be manually selected. If this is done, the product 

documentation (Section 9) should adequately explain to the consumer 

when the product is most likely to successfully synchronize.

4.B.1.Amount of Time Allotted to a Synchronization Attempt

We recommend that WWVB RCCs attempt to decode time codes 

for at least five consecutive minutes before determining that a 

synchronization attempt has failed.

4.B.2.Status of Display During a Synchronization Attempt

During the synchronization attempt it may be necessary to partially 

disable or alter the RCC’s display. For example, the second hand on 

an analog clock might stop, or the display on a digital clock 

might “flash” or show a message indicating that it is now receiving a 

radio signal. However, it is very important that the RCC continues 

to keep time during the synchronization attempt. If the attempt fails, 

the clock can then restore the time display and retain the same accuracy 

that it had prior to the failed attempt.

4.C.Synchronization by Radio at a Time Selected by the User

All WWVB RCCs should include a button or control that allows the 

user to attempt immediate synchronization, without waiting for the next 

scheduled synchronization. The RCC will attempt to decode the incoming 

signal whenever this control is activated. The RCC display should 

indicate whether the synchronization attempt succeeded or failed.

This button or control should be designed so that it cannot be activated by 

accident if the RCC is bumped or jostled. To avoid accidental activation, 

it might be desirable to use a button that needs to be held in place for 

several seconds before the clock attempts synchronization. 

4.D. Manual Clock Synchronization

All WWVB RCCs should allow the user to disable radio controlled 

timekeeping functions, so they can be operated as conventional 
15

clocks if necessary. This means that the consumer should be allowed 

to manually synchronize the time and date settings if the signal is 

unreceivable. This protects the RCC from becoming obsolete if it is 

moved to an area outside of the signal range. The display should indicate 

if the RCC is being operated without radio control.

4.E. Synchronization Indicator

All WWVB RCCs should indicate whether they have recently 

synchronized. Since the clocks are radio controlled and advertised as 

accurate, they are trusted by consumers, who typically assume that the 

displayed time is exactly right. However, this will not be true if the 

clock has not received the signal for a long period. All manufacturers 

should realize that it is extremely important to communicate to the person 

viewing the clock that the time can be trusted. This requires indicating 

whether or not the RCC has been recently synchronized.

Ideally, the RCC should be able to display the date and time of the 

last synchronization. If this is not possible, the RCC should indicate 

in some fashion whether it has been more than 24 hours since the 

last synchronization or, preferably, the total length of time (probably 

expressed in days) since the last synchronization. Since manufacturers 

should expect their products to work when used within the coverage area, 

Clock Synchronization ♦

Receive Time

Receive Date

Figure 7. RCC display indicating the date and time of the last synchronization.
♦ WWVB Radio Controlled Clocks

16

it is more appropriate for RCCs to indicate when synchronization has 

failed in the last 24 hours (an abnormal condition), than it is to indicate 

when synchronization has succeeded (a normal condition). Examples 

of appropriate synchronization indicators for digital and analog clocks 

are provided in Sections 4.E.1. and 4.E.2.

Some manufacturers may elect to only alert the consumer if 

synchronization has not occurred in a period longer than 24 hours; 

for example, 48 hours or 72 hours could be used. However, all 

manufacturers should attempt to meet the Section 3 requirement of 

keeping time between synchronizations to within ±0.5 s of UTC(NIST). 

A longer interval should be used only if the local oscillator (Section 8.C) 

is capable of keeping time to within ±0.5 s of UTC(NIST) throughout 

the entire interval. For this reason, synchronization intervals longer 

than 24 hours might not be acceptable for many RCC products.

4.E.1.Digital Clock Synchronization Indicator

There are numerous ways that a RCC with the ability to display 

alphanumeric characters or symbols can provide a synchronization 

indicator. All are acceptable if they are clearly explained and 

understandable to the consumer. Figure 7 shows a watch displaying the 

date and time of the last synchronization. This is the preferred method 

since it passes the most information along to the consumer.

4.E.2.Analog Clock Synchronization Indicator

If an analog clock does not have a digital inset, a synchronization 

indicator should still be provided. For example, the indicator can be a 

light that is illuminated when synchronization has not occurred within 

the last 24 hours. Or advancing the second hand every two seconds 

instead of every second could be used to indicate that the clock has not 

synchronized in the last 24 hours. Other methods can be used, including 

the use of audio beeps or tones. 
17

4.F. Adjustment of Display to Compensate for Delays 

Introduced During Synchronization

We recommend that manufacturers compensate for delays introduced 

during the synchronization of the display. For example, a RCC will 

require some processing time to decode the time code and to send the 

information to the display. The response time of the display will then 

introduce additional delays, which can be particularly large in the case 

of analog clocks that use a stepping motor. In some cases, the combined 

processing/response delays will exceed 0.1 s and become noticeable. 

The manufacturer should attempt to measure these delays and to 

advance the displayed time to compensate for the synchronization 

delays. 

Manufacturers and consumers should also be aware of the path delay 

as the signal travels from WWVB to the RCC. However, this delay is 

relatively small; radio signals travel at the speed of light, and the path 

delay should never exceed 0.02 s within the WWVB coverage area. 

For this reason, and because path delay cannot be estimated without 

knowing the RCC’s location, no path delay compensation is necessary. 

Time Zone Settings ♦

19

5. RECOMMENDED PRACTICES FOR TIME ZONE

 SETTINGS

WWVB broadcasts UTC as opposed to local time. Therefore, each RCC must 

have a time zone switch or control that allows the local time zone to be selected 

in order for the clock to display local time. 

5.A.Time Zone Selection

As a minimum requirement, all WWVB products should be capable 

of setting to the seven time zones listed in Table 3, so they can 

adequately service all potential consumers in the United States. 

However, since some WWVB products will be used through manual 

synchronization outside the coverage area (Section 4.D) or are capable 

of receiving other time signal stations (Section 3.D), we recommend that 

the time zone settings include each of the 38 time zones listed in Table 4. 

Both tables include the name, letter designation, and abbreviation for 

each time zone (when information is available), as well as the offset in 

hours from UTC.

Please note that if a RCC has the ability to select time zones offset by 

±12 hours from UTC in half-hour increments, then nearly all of the time 

zones in the world will be represented. Figure 8 provides a world time 

zone map; Figure 9 provides time zone information for the United States. 

Manufacturers should consider including similar maps in their product

documentation (Section 9).

Time Zone Settings ♦
♦ WWVB Radio Controlled Clocks

20

Table 3: Necessary Time Zone Options for WWVB RCC Products

UTC

Offset Non-United States

(hours) Letter Abbreviation Name Areas Include

–10:00 W HST or Hawaii–Aleutian Central French

 HAST Standard Time Polynesia, Tokelau, 

 Cook Islands, Tahiti,

 Johnston Atoll

–9:00 V AKST Alaska Gambier Islands

 Standard Time (French Polynesia)

–8:00 U PST Pacific Western Canada,

 Standard Time North Baja Peninsula

–7:00 T MST Mountain West Central Canada, 

 Standard Time South Baja Peninsula,

 Central and

 Western Mexico

–6:00 S CST Central Mexico, Easter Island,

 Standard Time Galapagos Islands,

 Central Canada,

 Central America

–5:00 R EST Eastern Western South America,

 Standard Time Cuba, Bahamas, Haiti,

 Jamaica, East Central

 Canada, Panama

–4:00 Q AST Atlantic Central South America,

 Standard Time Dominican Republic,

 Eastern Canada, Puerto 

 Rico, West Greenland, 

 Bermuda

 
21

Table 4: Recommended Time Zone Options for WWVB RCC Products Sold 

Internationally

UTC

Offset United States

(hours) Letter Abbreviation Name Other Areas

–12:00 Y International

 Date Line West

–11:00 X SST Samoa Midway Islands

 Standard Time

–10:00 W HST or Hawaii-Aleutian Central French

 HAST Standard Time Polynesia, Tokelau,

 Cook Islands,

Tahiti, Johnston Atoll

–9:30 — Marquesas Islands

 (French Polynesia)

–9:00 V AKST Alaska Gambier Islands

 Standard Time (French Polynesia)

–8:00 U PST Pacific Western Canada,

 Standard Time North Baja Peninsula

–7:00 T MST Mountain West Central Canada,

 Standard Time South Baja Peninsula,

Central and

Western Mexico

–6:00 S CST Central Mexico, Easter Island,

 Standard Time Galapagos Islands,

Central Canada,

Central America

–5:00 R EST Eastern Western South America,

 Standard Time Cuba, Bahamas, Haiti,

Jamaica, East Central

Canada, Panama

–4:00 Q AST Atlantic Central South America,

 Standard Time Dominican Republic,

Eastern Canada, Puerto 

Rico, West Greenland,

Bermuda

Time Zone Settings ♦
♦ WWVB Radio Controlled Clocks

22

Table 4: Recommended Time Zone Options for WWVB RCC Products Sold 

 Internationally (continued)

UTC

Offset United States

(hours) Letter Abbreviation Name Other Areas

–3:30 — Newfoundland Canada

–3:00 P Eastern South America,

 Central Greenland

–2:00 O Pernambuco (Brazil)

–1:00 N Azores, East Greenland

 (Svalbard and Jan

Mayen), Cape Verde

0 Z Western Europe, Iceland,

 West Africa, Canary

Islands, Coordinated

Universal Time,

Greenwich Mean Time

+1:00 A Central Europe

 (including France and

Spain), West Central

Africa, Norway,

Sweden, Denmark

+2:00 B Eastern Europe, Russia

 Zone 1, East Central

Africa, Turkey, Syria,

Jordan, Greece, Cyprus,

Israel, Lebanon, Finland

+3:00 C Russia Zone 2, East

 Africa, Iraq, 

Saudi Arabia,

Madagascar, Somalia,

Sudan, Kuwait,

Uganda, Yemen

+3:30 — Iran
23

Table 4: Recommended Time Zone Options for WWVB RCC Products Sold 

 Internationally (continued)

UTC

Offset United States

(hours) Letter Abbreviation Name Other Areas

+4:00 D Russia Zone 3,

 Georgia, Oman, Reunion,

Mauritius, Seychelles,

Azerbaijan, Armenia,

United Arab Emirates

+4:30 — Afghanistan

+5:00 E Russia Zone 4, British

 Indian Ocean Territory

(Chagos), Kerguelen

Island, Maldives Islands,

Turkmenistan, 

Tajikistan, Pakistan,

Uzbekistan,Western 

 Kazakhstan

+5:30 — India, Sri Lanka

+5:45 — Nepal

+6:00 F Russia Zone 5, Eastern

 Kazakhstan, Bangladesh,

Bhutan, Kyrgyzstan

+6:30 — Cocos Islands, Burma

+7:00 G Russia Zone 6, Western

 Indonesia, Southeast 

Asia

+8:00 H Russia Zone 7, Western

 Australia, China,

Hong Kong, Malaysia,

Philippines, Central

Indonesia, Singapore, 

Mongolia, Taiwan

Time Zone Settings ♦
♦ WWVB Radio Controlled Clocks

24

Table 4: Recommended Time Zone Options for WWVB RCC Products Sold 

 Internationally (continued)

UTC

Offset United States

(hours) Letter Abbreviation Name Other Areas

+9:00 I Russia Zone 8, Japan,

 Korea, Palau,

Eastern Indonesia

+9:30 — Central Australia

+10:00 K ChST Chamorro Russia Zone 9, Eastern 

 Standard Time Australia, Chamorro

(Guam and N. Mariana

Islands), Micronesia,

Papua New Guinea

+10:30 — Lord Howe Island

+11:00 L Russia Zone 10, Vanuatu,

 Solomon Islands,

New Caledonia,

East Micronesia Islands

+11:30 — Norfolk Island

+12:00 M Russia Zone 11,

 New Zealand, Fiji,

Tuvalu, Marshall Islands,

Nauru, Wake Island,

Wallis and Futuna,

Gilbert Islands (Kiribati), 

International

Date Line East

+12:45 — Chatham Islands

+13:00 — Tonga, Phoenix

 Islands (Kiribati)

+14:00 — Line Islands (Kiribati)
25

Time Zone Settings ♦ Figure 8. World time zone map.
♦ WWVB Radio Controlled Clocks

26

Figure 9. United States time zone map.
27

Daylight Saving Time (DST) ♦

6. RECOMMENDED PRACTICES FOR 

 DAYLIGHT SAVING TIME (DST)

The WWVB time code (Figure 1) includes information that tells the RCC 

whether standard time (ST) or daylight saving time (DST) is currently in 

effect, and also whether the current day is a transition day from ST to DST, 

or from DST to ST. All RCC products should decode this information so 

that consumers do not have to reset their clocks on the day of a time change. 

Figure 9 shows the areas within the United States that currently observe DST.

6.A.Handling of Transition Days

RCCs in areas that observe DST should advance one hour at 2 a.m. 

local time on the second Sunday of March each year, and move back one 

hour at 2 a.m. local time on the first Sunday of November of each year.

[3] 

RCCs must properly interpret the information in the time code (Figure 1) 

and apply the time zone settings (Section 5), so the transition takes place 

at exactly 2 a.m. local time. Special attention to the DST code is 

required when implementing this feature as some manufacturers have 

misinterpreted the code. It might be necessary for analog clocks to 

move forward 11 hours (rather than back one hour) on the transition 

from DST to ST, if their clock mechanisms do not allow the hands to 

be moved backwards.

Manufacturers might elect to design products that implement the DST 

rule[3] at the assigned time, even if the RCC has been unable to recently 

read the DST information in the WWVB time code. This will allow the 

RCC to handle the DST transition even if the signal has not been recently 

received. However, if the DST rules change (they were changed in 1986 

and 2007 and will probably change again), clocks programmed to follow 

them will fail, whereas the WWVB time code will always comply with 

the current rules. Therefore, we recommend that the DST information in 

the time code be used whenever possible and that the programmed rule 

only be used for backup or verification.

Another less important issue is that WWVB might be received under 

optimal conditions in areas (including South American countries such 

as Chile) that do not follow the same DST rules as the United States. This 

means RCCs located in these regions would be wrong at certain times of 

the year, regardless of whether the WWVB time code or the programmed 

DST rule for the United States was used to select the DST transition 

days. If a manufacturer sells products to consumers in these areas, we 

recommend that they allow the transition days and times to 
♦ WWVB Radio Controlled Clocks

28

and from DST to be selected by the consumer through some type of 

user interface. Inclusion of this feature also guards against product 

obsolescence if the DST rules in the United States are changed again.

6.B.Disabling/Enabling DST Switch

Certain regions of the United States do not observe DST (Figure 9). 

Therefore, all RCC products must provide the consumer with the option 

to disable DST so that clocks located in those regions remain on ST 

throughout the year. Care should be taken to inform the consumer that 

this is normally a one-time setting; DST should be disabled only if their 

area does not observe DST, and not simply if ST is currently in effect.

6.C.DST Indicator

We recommend that the RCC has a display or switch setting that informs 

the consumer whether DST or ST is currently in effect.
29

Leap Seconds, Leap Years, and the Two-Digit Code ♦

7. RECOMMENDED PRACTICES FOR LEAP SECONDS, 

 LEAP YEARS, AND THE TWO-DIGIT YEAR CODE

This section discusses the handling of miscellaneous time code settings, in 

particular the handling of leap seconds, leap years, and the two-digit year code.

7.A.Handling of Leap Seconds

When necessary (typically less than once per year), leap seconds are 

inserted into the UTC time scale on June 30th and/or December 31st. 

This keeps UTC within ±0.9 s of an astronomical time scale called UT1. 

The WWVB time code (Figure 1) includes a leap second bit (transmitted 

at second 56) that indicates whether a leap second will occur at the end of 

the current month. This allows the RCC to automatically insert the leap 

second. When a leap second does occur, the final minute of the day has 

61 seconds. The UTC sequence looks like this:

23 hours, 59 minutes, 59 seconds

23 hours, 59 minutes, 60 seconds

0 hours, 0 minutes, 0 seconds

In order to properly display the occurrence of a leap second, digital RCCs 

must be capable of displaying a value of 60 in the seconds’ field so that 

leap seconds can be indicated, for example 11:59:60 p.m. Analog RCCs 

cannot display a minute containing 60 seconds; therefore the second hand 

must remain in the same position (pointed straight up) for two consecutive 

seconds to indicate that a leap second has elapsed.

7.B.Handling of Leap Years

The WWVB time code (Figure 1) contains a leap year indicator 

(transmitted at second 55) that indicates whether the current year is a leap 

year. All RCCs that display date information must decode the leap year 

indicator so that dates after February 28th are correctly displayed. For 

example, if the RCC assumes that a leap year is a non-leap year, 

it will display a date of March 1st on February 29th. 

The leap year rule is simple, and manufacturers can elect to design 

products that automatically know whether a year is a leap year by looking 

at the year code. However, since the WWVB time code uses two-digit 

year codes (Section 7.C), these must first be converted to four digits 

before testing to see whether the year is a leap year, due to the problem 
♦ WWVB Radio Controlled Clocks

30

with centurial years (years that end with 00). Year numbers that are 

evenly divisible by 4 are leap years, with the exception of centurial 

years, which must be evenly divisible by 400.[4] Thus, 2000 was a leap 

year, whereas 2100, 2200, and 2300 will not be leap years. It is highly 

unlikely that the leap year rule will change, but in the event that it did, we 

recommend that the leap year information in the time code be used 

when possible. If the leap year rule is programmed into the product, 

we recommend that it be used only to backup or verify the time 

code information.

7.C. Handling of Two-Digit Year Code

The WWVB time code (Figure 1) includes only two digits of year 

information, so year information is ambiguous to the century. For 

example, the year 2004 is represented by “04.” This is generally not a 

problem for most RCCs since a twenty-first century consumer viewing the 

clock will intuitively know that “04” means 2004. However, if the RCC 

is designed to interface with other devices, such as computer systems, the 

manufacturer should convert the year information to four digits. Simply 

adding 2000 to the two-digit year code will keep the 

four-digit year correct until the year 2100. While this seems like a 

good solution in the early part of the 21st century, other solutions 

should be sought if the RCC is expected to output the correct year 

information indefinitely. 
31

Hardware Specifications ♦

8. RECOMMENDED PRACTICES FOR HARDWARE 

 SPECIFICATIONS

This section discusses hardware specifications for the receiver, antenna, and 

local oscillator.

8.A. Receiver Specifications

Complete receiver specifications are beyond the scope of this document, 

but the minimum goal of the manufacturer should be to include a receiver 

and antenna sufficiently sensitive to work anywhere within the CONUS 

during the nighttime hours. We recommend that RCC products should 

be sensitive enough to successfully synchronize to signals from WWVB 

with a field strength of 50 μV/m, if the signal to noise ratio exceeds 20 

dB. The RF bandwidth of the receiver should be narrow, typically ±10 Hz 

or less.

We recommend that digital signal processing (DSP) firmware be included 

in the receiver design to improve the RCC’s ability to read the time code. 

The redundancy of the time code information (Figure 1) can be used to 

considerably improve reception. During a given hour, only the minute 

information in the time code changes from frame-to-frame (except during 

the rare hours when leap second, DST, or UT1 information happens to 

be inserted or deleted). Therefore, the time code normally changes from 

frame-to-frame in an entirely predictable fashion, which makes bit and 

frame averaging possible and desirable. When properly implemented, 

the use of bit and frame averaging can be more effective than increasing 

the sensitivity of the receiver, or increasing the field strength presented 

to the RCC by many decibels. 

8.B.Antenna Considerations

Although external antennas obviously can provide better reception, 

we recommend that antennas be embedded inside the casing of the 

RCC to make the form factor more attractive and to prevent the antenna 

and/or its connecting wires from being damaged when the device is 

moved. Wristwatch antennas should not be contained in the band, 

so that RCC watch bands can be replaced in the same manner as 

the bands of ordinary watches when they are damaged or worn out. While 

external antennas are not recommended for stand-alone clocks 

and watches, they might be desirable for RCCs embedded in other 

devices, such as appliances.
♦ WWVB Radio Controlled Clocks

32

8.C.Local Oscillator Specifications

As recommended in Section 4.B, all RCCs will attempt to synchronize 

at least once per day. Therefore, in order to meet the Section 3 

requirement of keeping time between synchronizations to within ±0.5 s of 

UTC(NIST), the local quartz crystal oscillator must keep time to within 

about 0.48 s (we allow an 0.02 s error for signal propagation and internal 

clock synchronization delays) during a typical synchronization interval of 

24 hours (86400 s). Therefore, the maximum allowable frequency 

offset of the quartz crystal oscillator, given a time change ∆t in a 

period T, can be calculated as:

If the RCC uses a crystal oscillator with a nominal frequency of 

32768 Hz, the maximum allowable frequency offset from nominal is 

about 0.18 Hz. If the synchronization period is shortened, for example 

if the RCC is able to synchronize every 12 h instead of every 24 h, these 

requirements are relaxed. Reducing the synchronization interval by a 

factor of 2 (from 24 h to 12 h) would double the maximum allowable 

frequency offset to 0.36 Hz. Care should be taken by the manufacturer to 

choose a quartz crystal oscillator that is accurate and stable enough to stay 

within its allowable tolerance over its normal operating temperature range, 

without requiring adjustment during the expected lifetime of the RCC.

Manufacturers may also choose to employ schemes that digitally 

compensate for the frequency offset of the quartz crystal, allowing 

the ±0.5 s/day specification to be met with a less stable oscillator.

8.D.Battery Powered RCCs

If a RCC product is battery powered, we recommend that a “low 

battery” indicator be included on the display, so the consumers are 

aware that the battery or batteries need to be changed. When the 

voltage of a battery-powered device drops below a certain threshold 

determined by the manufacturer, the clock should stop completely 

rather than attempt to keep time with an insufficient power supply.
33

Product Documentation ♦

9. RECOMMENDED PRACTICES FOR PRODUCT 

 DOCUMENTATION

Although WWVB RCCs are less complicated than many other consumer 

electronic products, they do require more documentation than conventional 

clocks. An instruction sheet or manual that describes how to 

use all of the product’s features should be included with every product sold. 

We also recommend that all buttons and controls on RCC products be clearly 

labeled; for example, identify the button(s) or control(s) used to change time 

zones. The documentation (preferably on the packaging itself) should indicate 

which time zones are supported by the product, and the approximate coverage 

area, so that consumers do not mistakenly buy products that won’t work in their 

area. When space allows, such as on the back of a wall clock, it is helpful to 

engrave or stamp a condensed instruction sheet on the product itself. 

Manufacturers are also encouraged to include text in their product 

documentation that describes how to troubleshoot RCC reception problems. 

Some examples are provided in Section 11 of this guide.

In numerous cases, consumers have complained that the loss of an 

instruction manual has made their product unusable. Therefore, we 

recommend that all instruction manuals for current and past models be 

made available to consumers on-line, where they can be downloaded free 

of charge if necessary. 

9.A. Mention of NIST

We recommend that instruction manuals for WWVB RCC products 

mention that National Institute of Standards and Technology (NIST) 

radio station WWVB, located in Fort Collins, Colorado, is the source of 

the time signal received by the clock. It also should be noted that NIST 

is an agency of the United States government that provides official time 

to the United States. If manufacturers wish to provide a point of contact 

for obtaining more information about WWVB, they should reference 

the NIST Time and Frequency Division web site at http://tf.nist.

gov. However, manufacturers should not direct consumers to NIST for 

technical support since NIST is unable to provide it.
♦ WWVB Radio Controlled Clocks

34

9.B.Use of “Atomic Clock” Nomenclature

Many WWVB RCC products are labeled (on the product itself or 

in the documentation) as “atomic clocks.” This is probably seen 

by manufacturers as a useful marketing tool intended to capture the 

imagination of potential customers, and some might argue that it 

is appropriate since atomic clocks are located at the WWVB radio 

transmitter site. However, we contend that use of the term “atomic 

clock” is technically incorrect and misleading to consumers, and its usage 

should be avoided. Unless there is actually an atomic oscillator inside the 

RCC (such as a cesium or rubidium oscillator), we recommend that the 

term “radio controlled clock” be used to correctly describe the product. 

Labeling products or documentation with the term “atomic timekeeping” 

is also considered acceptable.
35

10. COMPLIANCE CHECKLIST

The checklist below is included to assist both manufacturers and consumers 

of RCC products. By answering the questions in the table, it can quickly 

be determined whether or not a given product complies with the 

recommendations provided in this handbook. 

 Product Complies

 with 

 Recommendation

 (Preferred Answer

 Is Always Yes)

Section Question Yes No

3 Is time always displayed to within 

 ±0.5 s during the entire interval 

 between clock synchronizations? N 

3 Is time always displayed to within 

 ±0.2 s during the entire interval 

 between clock synchronizations? O 

3 Does the clock display seconds? N 

3.A/3.B Does the clock have a label or icon 

 indicating that it is radio controlled? N 

3.A/3.B Does the clock display the date? O 

3.B If the clock is digital, does it 

 include a.m. and p.m. indicators? O 

3.B Was the clock designed without 

 an icon or picture of a satellite 

 dish on its display? O 

3.C Are the clock controls clearly

 labeled and situated? N 

3.D If the clock is capable of receiving 

 more than one time signal station, 

 does it indicate which station it 

 is receiving? O 

3.E Does the clock have a way to 

 indicate signal quality? O 

Compliance Checklist ♦

N = Necessary

O = Optional
♦ WWVB Radio Controlled Clocks

36

 Product Complies

 with 

 Recommendation

 (Preferred Answer

 Is Always Yes)

Section Question Yes No

3.E Is the signal quality meter not

 visible or disabled when the clock

 is not attempting to synchronize? O 

3.F Does the case have a marker 

 indicating the orientation of 

 the antenna? O 

4.A Does the clock continuously try 

to synchronize when it is first 

 turned on? N 

4.A Does the clock refrain from 

 displaying the time prior to its 

first synchronization? N

4.A Does the clock decode and 

 compare at least two consecutive

 time codes before determining

 that the initial synchronization 

 attempt is successful? N

4.A Can the clock synchronize 

 without having the display or 

 hands preset by the consumer? N 

4.B Does the clock attempt to 

 synchronize by radio at least 

 once every 24 hours? N 

4.B Does a clock that has been 

 previously synchronized continue 

 to keep time during a 

 synchronization attempt? N

4.B Does the clock attempt to 

 synchronize during the nighttime 

 hours when the signal from 

 WWVB is the strongest? N 

N = Necessary

O = Optional
37

 Product Complies

 with 

 Recommendation

 (Preferred Answer

 Is Always Yes)

 Section Question Yes No

4.B Does the clock attempt more 

 than one synchronization every 

 24 hours? O 

4.B Does the clock allow at least 

five minutes for a synchronization 

 attempt? N

4.C Does the clock include a 

 button or control that allows 

 the consumer to attempt to 

 synchronize at any time? N

4.D Can the clock be set manually,

 without radio synchronization? N 

4.E Does the clock include a

 synchronization indicator? N 

4.F Does the clock compensate for delays

 introduced during the 

 synchronization of its display? N

5.A Does the clock allow each of 

 the time zones listed in Table 3 

 to be selected? N 

5.A Does the clock allow each of 

 the time zones listed in Table 4 

 to be selected? O 

6 Does the clock automatically 

 adjust on the transition days from 

 ST to DST, and from DST to ST? N 

6.A Does the clock change from 

 standard time to DST at 2 a.m. 

 local time and vice versa? N 

Compliance Checklist ♦

N = Necessary

O = Optional
♦ WWVB Radio Controlled Clocks

38

 Product Complies

 with 

 Recommendation

 (Preferred Answer

 Is Always Yes)

Section Question Yes No

6.B Does the clock have a way to 

 disable DST for areas that do not 

 observe it? N 

6.C Does the clock include a DST 

 indicator? O 

7.A Does the clock properly handle

 leap seconds? O 

7.B If the clock displays date 

 information, does it properly 

 handle leap years? N

7.C If the clock displays four-digit 

 year information, does it properly 

 handle the two-digit year code? O

8.A Does the clock’s receiver meet

specifications?N

8.B Is the antenna concealed inside

 the clock unit? O 

8.C Does the local oscillator meet

specifications?N

8.D If the clock is powered by 

 batteries, is a “low battery” 

 indicator included? O 

9 In the case of wall clocks, are 

 condensed instructions included 

 on the product itself? O 

9 Does the manufacturer make 

 instructions manuals for the clock 

 available on-line? O 

N = Necessary

O = Optional
39

 Product Complies

 with 

 Recommendation

 (Preferred Answer

 Is Always Yes)

Section Question Yes No

9.A Does the product documentation

 mention NIST and WWVB? O 

9.B Does the product documentation 

 and/or labeling use the term “radio 

 controlled clock” or “atomic 

 timekeeping,” instead of the 

 incorrect “atomic clock?” O 

N = Necessary

O = Optional

Compliance Checklist ♦

41

11. RECOMMENDED PRACTICES FOR CONSUMERS 

 OF WWVB RCCS

The first and foremost reason that consumers purchase RCCs is accuracy. 

A RCC has a tremendous advantage over a conventional clock: when working 

properly, it is always right! Consumers never need to adjust a RCC, not 

even during the transition between daylight saving time and standard time. 

However, this section covers a few items that consumers need to know about to 

ensure that their RCC is working properly and providing the correct time.

11.A. How a WWVB RCC Works

WWVB RCCs are conventional quartz clocks with a miniature radio 

receiver inside that is permanently tuned to receive the 60 kHz signal 

from NIST radio station WWVB (Section 2). This station is located 

near Fort Collins, Colorado, about 100 km north of Denver. It broadcasts

a time signal continuously, 24 hours per day, 7 days per week, with 

a complete time message (called a time code) sent every minute. 

However, RCC products only attempt to read this time code 

periodically, often only once every 24 hours, typically during the night 

when the signal is strongest (Section 4.B).

The 60 kHz signal is located in a part of the radio spectrum called LF, 

which stands for low frequency. This is an appropriate name, because 

the FM radio and TV broadcasts that we are accustomed to listening 

to use frequencies thousands of times higher. The lowest frequency 

received by any other consumer radio is probably 530 kHz, the bottom 

of the AM broadcast band, and even that frequency is nearly nine times 

higher than the WWVB frequency. 

The 60 kHz signal does not provide enough bandwidth to carry audio 

information. Instead, all that is sent is a time code. The time code 

is simply a message containing time and date information. This 

information is sent in the form of binary digits, or bits, which have two 

possible values (0 or 1). Frame markers are also sent as part of the 

message, so the RCC can align the time code and read the bits in their 

proper order. The time code bits are generated by raising and lowering 

the power of the WWVB signals. They are sent at a very slow rate of 1 

bit per second, and it takes a full minute to send a complete time code 

or a message that tells the clock the current date and time. When a RCC 

is first turned on, it will probably miss the first time code, so it usually 

takes at least two minutes to synchronize, depending upon the signal 

quality and the receiver design.

Consumer Information ♦
♦ WWVB Radio Controlled Clocks

42

11.B. Time Zone Settings

Since the WWVB signal originates from a single location in Colorado, 

it does not contain any time zone information. Therefore, WWVB 

RCCs can not determine which time zone they are in unless this 

information is supplied by the consumer. Well-designed products 

(Section 5) allow the selection of all time zones where the clock 

could possibly be used.

The time broadcast by WWVB is Coordinated Universal Time (UTC), 

or the time kept at the Prime Meridian that passes through Greenwich, 

England. Clocks all over the world are synchronized to the same 

second as UTC in all cases and the same minute as UTC in nearly 

all cases.* However, the local hour is different than the UTC hour, 

based on the number of time zones between the local time zone and the 

Prime Meridian. While a few consumers want their clocks to display 

UTC (ham radio operators, for example), most prefer to display local 

time. WWVB RCCs apply a time zone correction of the UTC hour to 

convert UTC to local time. The size of this correction is shown in Table 

5 for the four major time zones in CONUS. 

When consumers move a WWVB RCC to another time zone, they need 

to change the time zone setting accordingly. Consumers that travel with 

RCCs should familiarize themselves with the procedure 

for changing time zones, so they can adjust their clocks whenever 

necessary.

* A few time zones (Table 4) differ from UTC by a non-integer number of 

hours (3.5 hours, for example). Clocks synchronized to local time in these 

regions will display a different minute than a clock synchronized to UTC, but 

the second will be the same.
43

Consumer Information ♦

Table 5: Difference between UTC and Local Time for the Four Major Time Zones 

in the CONUS

Difference from UTC Difference from UTC 

Time Zone During Standard Time During Daylight Time

Pacific–8 hours –7 hours

Mountain –7 hours –6 hours

Central –6 hours –5 hours

Eastern –5 hours –4 hours

11.C. Coverage Area of the WWVB Signal

During the nighttime hours, the WWVB signal is strong enough to 

synchronize clocks in the 48 states of the CONUS, in parts of Alaska 

and Hawaii, in all of Mexico, in most of the populated areas of Canada, 

and in some regions of Central and South America. (For coverage 

maps and signal strength information recorded at various sites, see 

http://tf.nist.gov)

The size of the coverage area is estimated using a field strength 

figure of 100 μV/m, which in theory is more than enough signal for a 

well-designed RCC to synchronize (Section 8.A). However, in practice, 

simply having a large signal doesn’t mean that a RCC will be able to 

work. What really matters is the signal-to-noise ratio, or the size of 

the signal compared to the size of the electrical noise near the same 

frequency. Raising the noise level is just as harmful as reducing 

the signal level. For example, if the RCC clock is near a source of 

interference, the noise level increases, and the clock might not be able 

to synchronize even if the local field strength of the time signal is high. 

Potential sources of interference are discussed in Section 11.D.1.
♦ WWVB Radio Controlled Clocks

44

11.D. General Troubleshooting Tips for WWVB RCCs

WWVB RCC products have different specifications, and use different 

controls and user interfaces, so technical support must be provided by 

the manufacturer and not by NIST. We recommend that consumers 

save the instruction sheets that come with their clocks, so they can 

refer to them in the future if necessary. Having said that, this section 

offers a few general tips for consumers whose RCCs aren’t displaying 

the correct time.

Nearly all problems reported by consumers with WWVB RCCs are 

related to the clock itself and not to the WWVB broadcast. Consumers 

should be aware that RCC problems caused by the WWVB broadcast 

are extremely rare. WWVB has a number of safeguards in place 

to help ensure that the correct time is always being broadcast, 

and time is kept at the station to within 100 nanoseconds of UTC.[2] 

The station does occasionally have signal outages, and all outages 

over five minutes in length are listed at http://tf.nist.gov. However, 

most outages are maintenance related and occur in the daytime hours 

when RCCs are not attempting to synchronize, so they have no effect on 

consumer products. Unplanned outages during the nighttime hours are 

responded to as quickly as possible and rarely last for more than 1 or 2 

hours. While it is possible that one of these outages can cause a RCC to 

miss one daily synchronization period, it is highly unlikely that this will 

happen two days in a row. Field strength varies due to the time of year 

and the current weather conditions, but it should be sufficient for RCCs 

in the CONUS to synchronize during each night of the year.

Consumers who suspect that their WWVB RCC is not displaying the 

correct time can check it by comparing it to other NIST time services, 

including the NIST web clock http://time.gov or the audio time signals 

from NIST radio stations WWV and WWVH.[1] The audio 

time signals can be heard using a shortwave radio or by telephone 

(dial 303– 499– 7111). WWVB RCCs should be within ±0.5 s of either 

source. Please note that the NIST web clock allows time zone selection 

and displays local time, but consumers must convert from UTC to get 

local time from WWV or WWVH (Table 5). 
45

Consumer Information ♦

11.D.1. General Troubleshooting Tips for RCCs That Won’t 

Synchronize at All

If their RCC won’t synchronize, we recommend that consumers try 

the following:

• If the RCC uses batteries, check them and replace if necessary. 

Low batteries can cause a variety of RCC problems. If the RCC 

used to work, but doesn’t work now, try changing the batteries 

before deciding it has failed.

• If you have a desktop RCC, try rotating it 90°. If you have a 

wall clock, try mounting it on a wall perpendicular to the one it is 

currently on (e.g., if it is on a north–south wall try an east–west 

wall). The antennas are directional, and reception can be improved 

by turning the antenna. If your RCC has a signal quality indicator 

and antenna orientation markers (Sections 3.E and 3.F), use them 

to help determine the proper antenna orientation.

• Place the RCC along a wall or near a window that faces 

Fort Collins, Colorado. 

• If you are staying in a hotel and traveling with a WWVB alarm 

clock or wristwatch, it will probably work best if you leave it near 

the window overnight. 

• If you have a WWVB wristwatch, remove it from your wrist 

at night so that it is motionless during the synchronization period 

(Section 4.B).

• Locate the clock away from the potential sources of interference 

listed in Table 6.
♦ WWVB Radio Controlled Clocks

46

Table 6: Potential Sources of Interference for WWVB RCCs

Source of 

interference Reason for interference

Computer monitors Some monitors have a scan frequency at or 

and televisions near the WWVB carrier frequency of 60 kHz. 

Place RCCs at least 1 to 2 meters away from 

computer monitors or televisions for best results.

Metal or Buildings made out of metal (such as mobile 

ferroconcrete homes) or buildings with metal roofs or steel siding 

buildings might prevent the clock from working by blocking 

or weakening the incoming signal. Ferroconcrete 

buildings (where the concrete has metal added to 

provide extra support), can also interfere with the 

signals. Place the clock near a window to give it 

the best chance of synchronizing.

Refrigerators, Electric motors can generate radio frequency 

air conditioners, interference (RFI) at the AC line frequency of 

household appliances, 60 Hz, which is a subharmonic of the 60 kHz 

or other devices with WWVB carrier. Place the RCC at least 1 or 

electric motors 2 meters away from these devices for best results.

Basements or WWVB signals can reach underground locations 

underground locations better than higher frequency signals, but the signal 

quality will be lessened if the RCC is placed in a 

basement. Also, basement walls are often made 

of ferroconcrete material (see above). Try placing 

the clock above the ground if it doesn’t work in a 

basement location.

Neon or Neon or fluorescent lights can sometimes emit RFI 

fluorescent lights that interferes with RCCs. If consumers suspect 

that lights are producing RFI, they can perform a 

simple test with a battery-powered AM radio. 

The radio should be tuned to a dial location 

between stations so that only noise is heard.

 
47

Consumer Information ♦

Table 6: Potential Sources of Interference for WWVB RCCs (continued)

Source of 

interference Reason for interference

Neon or If possible, turn off the lights to see if this noise 

fluorescent lights level goes down. If it is not possible to turn off 

(continued) the light, walk towards the light with the radio to

see if the noise increases. Try placing the RCC 

where the noise level is lowest to see if that helps. 

Electrical storms Electrical storms can generate RFI in the part of 

the spectrum used by WWVB. Lightning along 

the path between the consumer’s receiver and 

Fort Collins, Colorado, can potentially cause an 

RCC to miss a scheduled synchronization, but 

the problem should cease when the storm is over.

Overhead electrical The 60 Hz RFI emitted by electrical wires or other 

equipment power generating equipment can be a problem 

related to electrical since it is a subharmonic of the 60 kHz WWVB 

power generation carrier. Try moving the RCC as far from the 

RFI source as possible.

Broadcasting stations RCCs located near a radio or television station 

can be overloaded by the local signal. This can 

be a particular problem if 60 kHz is a subharmonic 

of the local carrier. 

If nothing else works, we recommend that consumers take the clock 

outdoors after dark and power it down (remove the batteries or 

unplug it), then power it up again to force it to look for the WWVB 

signal. If it works outdoors but not indoors, there is probably a local 

interference problem inside the building where the clock is located. 

If it doesn’t work outdoors at night, it’s probably best to return it and 

try a different model. 

Keep in mind that not all WWVB RCCs are created equal. Consumers

who think their clock is defective or that it is simply unable to 

work at their location should ask the manufacturer or dealer for a 

replacement. 
♦ WWVB Radio Controlled Clocks

48

11.D.2. General Troubleshooting Tips for RCCs Off by 

One Hour or More

Remember, minutes and seconds are the same in nearly all time 

zones, only hours are different. Therefore, if a clock is off by exactly 

one hour or by a multiple of exactly one hour, it probably has to do 

with a time zone setting. Consumers should make sure that they have 

properly selected their local time zone using the instructions 

that came with the RCC. If the clock does not have a setting for the 

consumer’s local time zone (Section 5.A), we recommend that they 

return it to the dealer. Consumers outside the CONUS should make

sure that a RCC product handles their time zone prior to purchasing it.

Consumers who live in areas that do not observe Daylight Saving 

Time (Figure 9) must make sure that DST is disabled on their RCC 

(Section 6.B). If the RCC lacks this feature, consumers might still be 

able to select another time zone to make the RCC display the correct 

time when DST is in effect.

11.D.3. General Troubleshooting Tips for RCCs Off by a 

Few Minutes or Seconds

Properly working and designed RCCs should display time accurate 

to within ±0.5 s or better (Section 3). However, a malfunctioning 

RCC can be off by a few seconds or even minutes, for the reasons 

listed below:

•	 Reception Problem — If a RCC isn’t receiving the signals from 

WWVB at least once per day, the time will “drift” and gradually get 

further and further from the correct time. Remember, if WWVB 

isn’t being received, the clock is no longer radio controlled, it’s just 

a conventional quartz clock. Its accuracy will then depend on the 

quality of the quartz crystal (Section 8.C). Most quartz clocks can 

keep time to 1 s per day or better, but some could 

be off by several seconds per day and a time error always 

accumulates between synchronizations. Consumers are 

advised to purchase only RCCs with synchronization indicators 

(Section 4.E), so they will know whether or not the clock 

has recently synchronized. If the product does not have a 

synchronization indicator and the consumer cannot tell if the 

signal is being received, we recommend powering down the 

clock (by unplugging it or removing the batteries), then powering 

it up again to see if it can synchronize. If it doesn’t, see 

Section 11.D.1 for tips on improving reception.
49

•	 Alignment Problem — Some consumers might obtain analog 

RCCs whose hands aren’t properly aligned. This could cause 

the clock to be off by 1 s or more even if it is receiving the signal 

properly. The clock might not have been properly aligned at the 

factory, or it might have been jostled during shipment, causing 

the hands to move. Some manufacturers explain how to align the 

hands on their instruction sheet. If the consumer is unable to do 

this, and if the small error bothers them, we recommend that they 

return the clock to the dealer for replacement.

•	 Parallax Problem — The parallax problem refers to the apparent 

shifting of an object when viewed at different angles and can 

prevent problems when viewing analog RCCs. When consumers 

check the accuracy of an analog RCC, they need to be sure they are 

looking straight at the clock face and not viewing it from an angle. 

Consumers who view the clock from an angle might think 

it is off by a few seconds even if it is not. This is similar to trying 

to read the speedometer from the passenger seat of a car and 

thinking the speed is faster or slower than it actually is.

11.D.4. General Troubleshooting Tips Concerning Daylight 

Saving Time (DST)

Problems related to DST are among the most common problems 

experienced by RCC consumers. If a RCC doesn’t change during 

the transition from DST to standard time, or vice versa, it probably 

means that it has not received the signal recently, so it didn’t know 

about the time change. See Sections 11.D.1 and 11.D.3 for tips on 

improving reception. 

If reception appears to be fine and the RCC didn’t change, 

consumers should make sure that DST is not disabled on their 

RCC (Section 6.B), if their area observes DST. Conversely, if the 

area does not observe DST and the RCC did change, consumers 

need to disable the DST setting.

Consumer Information ♦
♦ WWVB Radio Controlled Clocks

50
51

12. REFERENCES

[1] Lombardi, M.A. “NIST Time and Frequency Services.” Natl. Inst. Stand. 

Technol. Spec. Publ. 432, 71 p., January 2002.

[2] Nelson, G.K., M.A. Lombardi, and D.T. Okayama. “NIST Time and 

Frequency Radio Stations: WWV, WWVH, and WWVB.” Natl. Inst. 

Stand. Technol. Spec. Publ. 250–67, 156 p., January 2005.

[3] 15 United States Code 6(IX)(260a).

[4] Seidelmann, P. Kenneth, ed. “Explanatory Supplement to the Astronomical 

Almanac.” University Science Books, pp. 580–581, 1992. 

References ♦
♦ WWVB Radio Controlled Clocks

52

August 2009