NISTIR 8044-09

NIST Time and Frequency Bulletin

Karen Hansen Lichtfuss, Editor

http://dx.doi.org/10.6028/NIST.IR.8044-09
NISTIR 8044-09

NIST Time and Frequency Bulletin

Karen Hansen Lichtfuss, Editor

Time and Frequency Division

Physical Measurement Laboratory

http://dx.doi.org/10.6028/NIST.IR.8044-09

September 2015

U.S. Department of Commerce 

Penny Pritzker, Secretary

National Institute of Standards and Technology 

Willie E. May, Under Secretary of Commerce for Standards and Technology and Director 
1

NIST TIME AND FREQUENCY BULLETIN

NIST IR 8044-09

No. 694 September 2015

1. GENERAL BACKGROUND INFORMATION........................................................................................... 2

2. TIME SCALE INFORMATION ............................................................................................................... 2

3. BROADCAST OUTAGES OVER FIVE MINUTES AND WWVB 

 PHASE PERTURBATIONS..................................................................................................................... 4

4. NOTES ON NIST TIME SCALES AND PRIMARY STANDARDS........................................................... 4

5. UTC (NIST) – AT1 PARAMETERS ..........................................................................................................5

This bulletin is published monthly. Address correspondence to: 

Karen Hansen Lichtfuss, Editor 

Time and Frequency Division 

National Institute of Standards and Technology

325 Broadway

Boulder, CO 8O3O5-3328 

(3O3) 497-3295

Email: karen.hansenlichtfuss@nist.gov

U.S. DEPARTMENT OF COMMERCE, Penny Pritzker, Secretary

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, Willie E. May, Under Secretary of Commerce for 

Standards and Technology and Director
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1. GENERAL BACKGROUND INFORMATION

_______________________________________________________________________________________________________________________________

ACRONYMS AND ABBREVIATIONS USED IN THIS BULLETIN

ACTS - Automated Computer Time Service

BIPM - Bureau International des Poids et Mesures

GPS - Global Positioning System

IERS - International Earth Rotation Service

MC - Master Clock

MJD - Modified Julian Date

NIST - National Institute of Standards and Technology ns - nanosecond

SI - International System of Units μs - microsecond

TA - Atomic Time ms - millisecond

TAI - International Atomic Time s - second

USNO - United States Naval Observatory min - minute

UT1 - Universal Time (Astronomical)

UTC - Coordinated Universal Time

_______________________________________________________________________________________________________

2. TIME SCALE INFORMATION

 The values listed below are based on data from the IERS, the USNO, and NIST. The UTC(USNO,MC) - UTC(NIST) values 

are averaged measurements from all available common-view GPS satellites (see bibliography on page 5). UTC - UTC(NIST) 

data are on page 3.

 0000 HOURS COORDINATED UNIVERSAL TIME

August 2015 MJD UT1-UTC(NIST)

 (±5 ms)

UTC(USNO,MC) - UTC(NIST)

 (±20 ns)

6 57240 + 296 ms + 10 ns

13 57247 + 290 ms + 10 ns

20 57254 + 285 ms + 13 ns

27 57261 + 279 ms + 11 ns

 The master clock pulses used by the WWV, WWVH, and WWVB time-code transmissions are referenced to the UTC

(NIST) time scale. Occasionally, 1 s is added to the UTC time scale. This second is called a leap second. Its purpose is t o 

keep the UTC time scale w ithin ±0.9 s of the UT1 astronomical time scale, w hich changes slightly due to variations in the

Earth’s period of rotation.

 NOTE: No leap second w ill be added at the end of December 2015.

 Positive leap seconds, beginning at 23 h 59 min 60 s UTC and ending at 0 h 0 min 0 s UTC, w ere inserted in the UTC 

time scale on 30 June 1972, 1981-1983, 1985, 1992-1994, 1997, 2012, 2015 and on 31 December 1972-1979, 1987, 

1989, 1990,1995, 1998, 2005, 2008. 

 The use of leap seconds ensures that UT1 - UTC w ill alw ays be held w ithin ±0.9 s. The current value of UT1 - UTC is 

called the DUT1 correction. DUT1 corrections are broadcast by WWV, WWVH, WWVB, and ACTS and are printed below . 

These corrections may be added to the received UTC time signals in order to obtain UT1. 

DUT1 = UT1 - UTC =

+ 0.3 s beginning 0000 UTC 01 July 2015

-0.7 s beginning 0000 UTC 28 May 2015

-0.6 s beginning 0000 UTC 19 March 2015

 -0.5 s beginning 0000 UTC 25 December 2014

 -0.4 s beginning 0000 UTC 25 September 2014

 -0.3 s beginning 0000 UTC 08 May 2014

- 0.2 s beginning 0000 UTC 20 February 2014

- 0.1 s beginning 0000 UTC 21 November 2013

+ 0.0 s beginning 0000 UTC 22 August 2013

+ 0.1 s beginning 0000 UTC 11 April 2013

+ 0.2 s beginning 0000 UTC 31 January 2013

+ 0.3 s beginning 0000 UTC 25 October 2012

+ 0.4 s beginning 0000 UTC 01 July 2012

- 0.6 s beginning 0000 UTC 10 May 2012

- 0.5 s beginning 0000 UTC 09 February 2012

- 0.4 s beginning 0000 UTC 04 November 2011
DATE MJD UTC-UTC(NIST), ns

Jul 26, 2015 57229 9.4

Jul 16, 2015 57219 8.8

Jul 6, 2015 57209 7.7

Jun 26, 2015 57199 6.6

Jun 16, 2015 57189 6.3

Jun 6, 2015 57179 5.8

May 27, 2015 57169 5.6

May 17, 2015 57159 4.7

May 7, 2015 57149 4.8

Apr. 27, 2015 57139 4.4

Apr. 17, 2015 57129 4.4

Apr. 07, 2015 57119 4.5

Mar. 28, 2015 57109 4.3

Mar. 18, 2015 57099 4.3

Mar. 8, 2015 57089 4.2

Feb. 26, 2015 57079 3.5

Feb. 16, 2015 57069 2.5

Feb. 06, 2015 57059 2.5

Jan. 27, 2015 57049 3.5

Jan. 17, 2015 57039 5.0

Jan. 7, 2015 57029 5.8

Dec. 28, 2014 57019 7.8

Dec. 18, 2014 57009 9.6

Dec. 8, 2014 56999 10.2

Nov. 28, 2014 56989 11.1

Nov. 18, 2014 56979 11.6

Nov. 08, 2014 56969 12.2

Oct. 29, 2014 56959 10.9

Oct. 19, 2014 56949 10.6

Oct 9, 2014 56939 9.3

0000 Hours Coordinated Universal Time

The difference between UTC(NIST) and UTC has been within ±100 ns since July 6, 1994. The table below shows values

of UTC - UTC(NIST) as supplied by the BIPM in their Circular T publication for the most recent 310-day period in which

data are available. Data are given at ten-day intervals. Five-day interval data are available in Circular T . 

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4

3. BROADCAST OUTAGES OVER FIVE MINUTES AND WWVB PHASE PERTURBATIONS

OUTAGES OF 5 MINUTES OR MORE PHASE PERTURBATIONS

2 ms

Station August

2015

 MJD Began 

UTC

Ended 

UTC

Freq. August

2015 MJD Began

UTC

End

UTC

WWVB None None

WWV

None None

WWVH

None None

4. NOTES ON NIST TIME SCALES AND PRIMARY STANDARDS

 Primary frequency standards developed and operated by NIST are used to provide accuracy (rate) input to the BIPM and to 

provide the best possible realization of the SI second. NIST-F1 and NIST F-2, cold-atom cesium fountain frequency standards, have

served as the U.S. primary standards of time and frequency since 1999. The uncertainty of NIST-F2 is currently about 1 part in 1016. 

 The AT1 scale is run in real-time by use of data from an ensemble of cesium standards and hydrogen masers. It is a free-running 

scale whose frequency is maintained as nearly constant as possible by choosing the optimum weight for each clock that contributes 

to the computation.

 UTC(NIST) is generated as an offset from our real-time scale AT1. It is steered in frequency towards UTC by use of data 

published by the BIPM in its Circular T. Changes in the steering frequency will be made, if necessary, at 0000 UTC on the first day 

of the month, and occasionally at mid-month. A change in frequency is limited to no more than ±2 ns/day. The frequency of 

UTC(NIST) is kept as stable as possible at other times. 

 UTC is generated at the BIPM by use of a post-processed time-scale algorithm and is not available in real-time. The parameters 

that we use to generate UTC(NIST) in real-time are therefore based on an extrapolation of UTC from the most recent available data. 

References:

Allan, D.W.; Hellwig, H.; and Glaze, D.J., "An accuracy algorithm for an atomic time scale," Metrologia, Vol.11, No.3, pp. 133-138 

(1975). 

Allan, D.W.; Davis, D.D.; Weiss, M.A.; Clements, A.; Guinot, B.; Granveaud, M.; Dorenwendt, K.; Fischer, B.; Hetzel, P.; Aoki, S.; 

Fujimoto, M.; Charron, L.; and Ashby, N., “Accuracy of international time and frequency comparisons via global positioning system 

satellites in common-view,” IEEE Transactions on Instrumentation and Measurement, Vol. IM-34, pp.118-125 (1985).

Heavner, T.P.; Jefferts, S.R.; Donley; E.A.; Shirley, J.H. and Parker, T.E., “NIST F1; recent improvements and accuracy 

evaluations,” Metrologia, Vol. 42, pp. 411-422 (2005).

Jefferts, S.R.; Shirley, J.; Parker, T.E.; Heavner, T.P.; Meekhof, D.M.; Nelson, C., Levi, F.; Costanza, G.; De Marchi, A.; Drullinger, 

R.; Hollberg, L.; Lee, W.D.; and Walls, F.L., “Accuracy evaluation of NIST-F1,” Metrologia, Vol. 39, pp. 321-336 (2002).

Lewandowski, W. and Thomas, C., “GPS Time transfer,” Proceedings of the IEEE, Vol. 79, pp. 991-1000 (1991).

Parker, T.E.; Jefferts, S.R.; Heavner, T.P.; and Donley, E.A., “Operation of the NIST-F1 caesium fountain primary frequency 

standard with a maser ensemble, including the impact of frequency transfer noise,” Metrologia, Vol. 42, pp. 423-430 (2005).

Weiss, M.A.; Allan, D.W., “An NBS Calibration Procedure for Providing Time and Frequency at a Remote Site by Weighting and 

Smoothing of GPS Common View Data,” IEEE Transactions on Instrumentation and Measurement, Vol. IM-36, pp. 572-578 (1987).
Month xls

(s)

x

(ns)

y

(ns/d)

 T0

(MJD)

Valid until 0000 on:

(MJD)

Nov 15 -36 -424074.25 -36.6* 57327 57357

Oct 15 -36 -422939.65 -36.6 57296 57327*

Sep 15 -36 -421832.65 -36.9 57266 57296

Aug 15 -36 -421057.75 -36.9 57245 57266

Aug 15 -36 -420686.75 -37.1 57235 57245†

Jul 15 -36 -419536.65 -37.1 57204 57235

Jun 15 -35 -418423.65 -37.1 57174 57204

May 15 -35 -417273.55 -37.1 57143 57174

Apr 15 -35 -416161.55 -37.1 57113 57143

Mar 15 -35 -415010.45 -37.1 57082 57113

Feb 15 -35 -413971.65 -37.1 57054 57082

Jan 15 -35 -412824.65 -37.0 57023 57054

Dec 14 -35 -412010.65 -37.0 57001 57023

Dec 14 -35 -411675.85 -37.2 56992 57001†

Nov 14 -35 -410931.85 -37.2 56972 56992

Nov 14 -35 -410557.85 -37.4 56962 56972†

Oct 14 -35 -409735.05 -37.4 56940 56962

Oct 14 -35 -409379.1 -37.55* 56931 56940†

Sep 14 -35 -408270.6 -37.55 56901 56931

Aug 14 -35 -407106.6 -37.55 56870 56901

Jul 14 -35 -405942.5 -37.55 56839 56870

Jun 14 -35 -404816 -37.55 56809 56839

† Rate change in mid-month

*Provisional value

The table below lists parameters that are used to define UTC(NIST) with respect to our real-time scale AT1. To find the

value of UTC(NIST) - AT1 at any time T (expressed as a Modified Julian Date, including a fraction if needed), the

appropriate equation to use is the one for which the desired T is greater than or equal to the entry in the T0 column and

less than the entry in the last column. The values of xls, x, and y for that month are then used in the equation below to

find the desired value. The parameters x and y represent the offsets in time and frequency, respectively, between

UTC(NIST) and AT1; the parameter xls is the number of leap seconds applied to both UTC(NIST) and UTC, as specified

by the IERS. Leap seconds are not applied to AT1.

5. UTC(NIST) – AT1 PARAMETERS

UTC(NIST) - AT1 = xls + x + y*(T -T0)

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