# hwclock

## NAME



           hwclock - time clocks utility
    


## SYNOPSIS



           **hwclock**[_function_] [_option_...]
    


## DESCRIPTION



           **hwclock** is an administration tool for the time clocks. It can:
           display the Hardware Clock time; set the Hardware Clock to a
           specified time; set the Hardware Clock from the System Clock; set
           the System Clock from the Hardware Clock; compensate for Hardware
           Clock drift; correct the System Clock timescale; set the kernel’s
           timezone, NTP timescale, and epoch (Alpha only); and predict
           future Hardware Clock values based on its drift rate.
    
           Since v2.26 important changes were made to the **--hctosys** function
           and the **--directisa** option, and a new option **--update-drift** was
           added. See their respective descriptions below.
    


## FUNCTIONS



           The following functions are mutually exclusive, only one can be
           given at a time. If none is given, the default is **--show**.
    
           **-a, --adjust**
               Add or subtract time from the Hardware Clock to account for
               systematic drift since the last time the clock was set or
               adjusted. See the discussion below, under **The Adjust Function**.
    
           **--getepoch** ; **--setepoch**
               These functions are for Alpha machines only, and are only
               available through the Linux kernel RTC driver.
    
               They are used to read and set the kernel’s Hardware Clock
               epoch value. Epoch is the number of years into AD to which a
               zero year value in the Hardware Clock refers. For example, if
               the machine’s BIOS sets the year counter in the Hardware Clock
               to contain the number of full years since 1952, then the
               kernel’s Hardware Clock epoch value must be 1952.
    
               The **--setepoch** function requires using the **--epoch** option to
               specify the year. For example:
    
               **hwclock --setepoch --epoch=1952**
    
               The RTC driver attempts to guess the correct epoch value, so
               setting it may not be required.
    
               This epoch value is used whenever **hwclock** reads or sets the
               Hardware Clock on an Alpha machine. For ISA machines the
               kernel uses the fixed Hardware Clock epoch of 1900.
    
           **--param-get=**_parameter_ ; **--param-set=**_parameter_ =_value_
               Read and set the RTC’s parameter. This is useful, for example,
               to retrieve the RTC’s feature or set the RTC’s Backup
               Switchover Mode.
    
               _parameter_ is either a numeric RTC parameter value (see the
               Kernel’s _include/uapi/linux/rtc.h_) or an alias. See **--help** for
               a list of valid aliases. _parameter_ and _value_ , if prefixed with
               0x, are interpreted as hexadecimal, otherwise decimal values.
    
           **--predict**
               Predict what the Hardware Clock will read in the future based
               upon the time given by the **--date** option and the information
               in _/etc/adjtime_. This is useful, for example, to account for
               drift when setting a Hardware Clock wakeup (aka alarm). See
               rtcwake(8).
    
               Do not use this function if the Hardware Clock is being
               modified by anything other than the current operating system’s
               **hwclock** command, such as '11 minute mode' or from dual-booting
               another OS.
    
           **-r** , **--show** ; **--get**
               Read the Hardware Clock and print its time to standard output
               in the **ISO 8601** format. The time shown is always in local
               time, even if you keep your Hardware Clock in UTC. See the
               **--localtime** option.
    
               Showing the Hardware Clock time is the default when no
               function is specified.
    
               The **--get** function also applies drift correction to the time
               read, based upon the information in _/etc/adjtime_. Do not use
               this function if the Hardware Clock is being modified by
               anything other than the current operating system’s **hwclock**
               command, such as '11 minute mode' or from dual-booting another
               OS.
    
           **-s** , **--hctosys**
               Set the System Clock from the Hardware Clock. The time read
               from the Hardware Clock is compensated to account for
               systematic drift before using it to set the System Clock. See
               the discussion below, under **The Adjust Function**.
    
               The System Clock must be kept in the UTC timescale for
               date-time applications to work correctly in conjunction with
               the timezone configured for the system. If the Hardware Clock
               is kept in local time then the time read from it must be
               shifted to the UTC timescale before using it to set the System
               Clock. The **--hctosys** function does this based upon the
               information in the _/etc/adjtime_ file or the command line
               arguments **--localtime** and **--utc**. Note: no daylight saving
               adjustment is made. See the discussion below, under **LOCAL vs**
               **UTC**.
    
               The kernel also keeps a timezone value, the **--hctosys** function
               sets it to the timezone configured for the system. The system
               timezone is configured by the **TZ** environment variable or the
               _/etc/localtime_ file, as tzset(3) would interpret them. The
               obsolete _tz_dsttime_ field of the kernel’s timezone value is
               set to zero. (For details on what this field used to mean, see
               settimeofday(2).)
    
               When used in a startup script, making the **--hctosys** function
               the first caller of settimeofday(2) from boot, it will set the
               NTP '11 minute mode' timescale via the
               _persistent_clock_is_local_ kernel variable. If the Hardware
               Clock’s timescale configuration is changed then a reboot is
               required to inform the kernel. See the discussion below, under
               **Automatic Hardware Clock Synchronization by the Kernel**.
    
               This is a good function to use in one of the system startup
               scripts before the file systems are mounted read/write.
    
               This function should never be used on a running system.
               Jumping system time will cause problems, such as corrupted
               filesystem timestamps. Also, if something has changed the
               Hardware Clock, like NTP’s '11 minute mode', then **--hctosys**
               will set the time incorrectly by including drift compensation.
    
               Drift compensation can be inhibited by setting the drift
               factor in _/etc/adjtime_ to zero. This setting will be
               persistent as long as the **--update-drift** option is not used
               with **--systohc** at shutdown (or anywhere else). Another way to
               inhibit this is by using the **--noadjfile** option when calling
               the **--hctosys** function. A third method is to delete the
               _/etc/adjtime_ file. **Hwclock** will then default to using the UTC
               timescale for the Hardware Clock. If the Hardware Clock is
               ticking local time it will need to be defined in the file.
               This can be done by calling **hwclock --localtime --adjust** ; when
               the file is not present this command will not actually adjust
               the Clock, but it will create the file with local time
               configured, and a drift factor of zero.
    
               A condition under which inhibiting **hwclock** 's drift correction
               may be desired is when dual-booting multiple operating
               systems. If while this instance of Linux is stopped, another
               OS changes the Hardware Clock’s value, then when this instance
               is started again the drift correction applied will be
               incorrect.
    
               For **hwclock** 's drift correction to work properly it is
               imperative that nothing changes the Hardware Clock while its
               Linux instance is not running.
    
           **--set**
               Set the Hardware Clock to the time given by the **--date** option,
               and update the timestamps in _/etc/adjtime_. With the
               **--update-drift** option also (re)calculate the drift factor. Try
               it without the option if **--set** fails. See **--update-drift**
               below.
    
           **--systz**
               This is an alternate to the **--hctosys** function that does not
               read the Hardware Clock nor set the System Clock; consequently
               there is not any drift correction. It is intended to be used
               in a startup script on systems with kernels above version 2.6
               where you know the System Clock has been set from the Hardware
               Clock by the kernel during boot.
    
               It does the following things that are detailed above in the
               **--hctosys** function:
    
               •   Corrects the System Clock timescale to UTC as needed. Only
                   instead of accomplishing this by setting the System Clock,
                   **hwclock** simply informs the kernel and it handles the
                   change.
    
               •   Sets the kernel’s NTP '11 minute mode' timescale.
    
               •   Sets the kernel’s timezone.
    
               The first two are only available on the first call of
               settimeofday(2) after boot. Consequently this option only
               makes sense when used in a startup script. If the Hardware
               Clocks timescale configuration is changed then a reboot would
               be required to inform the kernel.
    
           **-w** , **--systohc**
               Set the Hardware Clock from the System Clock, and update the
               timestamps in _/etc/adjtime_. With the **--update-drift** option
               also (re)calculate the drift factor. Try it without the option
               if **--systohc** fails. See **--update-drift** below.
    
           **--vl-read** , **--vl-clear**
               Some RTC devices are able to monitor the voltage of the backup
               battery and thus provide a way for the user to know that the
               battery should be replaced. The **--vl-read** function retrieves
               the Voltage Low information and decodes the result into
               human-readable form. The **--vl-clear** function resets the
               Voltage Low information, which is necessary for some RTC
               devices after a battery replacement.
    
               See the Kernel’s _include/uapi/linux/rtc.h_ for details on which
               pieces of information may be returned. Note that not all RTC
               devices have this monitoring capability, nor do all drivers
               necessarily support reading the information.
    
           **-h** , **--help**
               Display help text and exit.
    
           **-V** , **--version**
               Display version and exit.
    


## OPTIONS



           **--adjfile=**_filename_
               Override the default _/etc/adjtime_ file path.
    
           **--date=**_date_string_
               This option must be used with the **--set** or **--predict**
               functions, otherwise it is ignored.
    
               **hwclock --set --date='16:45'**
    
               **hwclock --predict --date='2525-08-14 07:11:05'**
    
               The argument must be in local time, even if you keep your
               Hardware Clock in UTC. See the **--localtime** option. Therefore,
               the argument should not include any timezone information. It
               also should not be a relative time like "+5 minutes", because
               **hwclock** 's precision depends upon correlation between the
               argument’s value and when the enter key is pressed. Fractional
               seconds are silently dropped. This option is capable of
               understanding many time and date formats, but the previous
               parameters should be observed.
    
           **--delay=**_seconds_
               This option overrides the default delay used when setting the
               clock time. The default is **0.5**(500 ms) for rtc_cmos; for
               other RTC types the delay is **0**. If the RTC type cannot be
               determined (from sysfs), then the delay defaults also to **0.5**
               to be backwardly compatible.
    
               The 500 ms default is based on the commonly used
               MC146818A-compatible (x86) hardware clock. This Hardware Clock
               can only be set to an integer time plus one half second. The
               integer time is required because there is no interface to get
               or set a fractional second. The additional half second is
               because the Hardware Clock updates to the following second
               precisely 500 ms after setting the new time. Unfortunately,
               this behavior is hardware specific and in some cases a
               different delay is required.
    
           **-D** , **--debug**
               Use **--verbose**. The **--debug** option has been deprecated and may
               be repurposed or removed in a future release.
    
           **--directisa**
               This option is meaningful for ISA compatible machines in the
               x86 and x86_64 family. For other machines, it has no effect.
               This option tells **hwclock** to use explicit I/O instructions to
               access the Hardware Clock. Without this option, **hwclock** will
               use the rtc device file, which it assumes to be driven by the
               Linux RTC device driver. As of v2.26 it will no longer
               automatically use directisa when the rtc driver is
               unavailable; this was causing an unsafe condition that could
               allow two processes to access the Hardware Clock at the same
               time. Direct hardware access from userspace should only be
               used for testing, troubleshooting, and as a last resort when
               all other methods fail. See the **--rtc** option.
    
           **--epoch=**_year_
               This option is required when using the **--setepoch** function.
               The minimum _year_ value is 1900. The maximum is system
               dependent (**ULONG_MAX - 1**).
    
           **-f** , **--rtc=**_filename_
               Override **hwclock** 's default rtc device file name. Otherwise it
               will use the first one found in this order: _/dev/rtc_ ,
               _/dev/rtc0_ , _/dev/misc/rtc_. For **IA-64:**_/dev/efirtc_
               _/dev/misc/efirtc_
    
           **-l** , **--localtime** ; **-u** , **--utc**
               Indicate which timescale the Hardware Clock is set to.
    
               The Hardware Clock may be configured to use either the UTC or
               the local timescale, but nothing in the clock itself says
               which alternative is being used. The **--localtime** or **--utc**
               options give this information to the **hwclock** command. If you
               specify the wrong one (or specify neither and take a wrong
               default), both setting and reading the Hardware Clock will be
               incorrect.
    
               If you specify neither **--utc** nor **--localtime** then the one last
               given with a set function (**--set** , **--systohc** , or **--adjust**), as
               recorded in _/etc/adjtime_ , will be used. If the adjtime file
               doesn’t exist, the default is UTC.
    
               Note: daylight saving time changes may be inconsistent when
               the Hardware Clock is kept in local time. See the discussion
               below, under **LOCAL vs UTC**.
    
           **--noadjfile**
               Disable the facilities provided by _/etc/adjtime_. **hwclock** will
               not read nor write to that file with this option. Either **--utc**
               or **--localtime** must be specified when using this option.
    
           **--test**
               Do not actually change anything on the system, that is, the
               Clocks or _/etc/adjtime_ (**--verbose** is implicit with this
               option).
    
           **--update-drift**
               Update the Hardware Clock’s drift factor in _/etc/adjtime_. It
               can only be used with **--set** or **--systohc**.
    
               A minimum four hour period between settings is required. This
               is to avoid invalid calculations. The longer the period, the
               more precise the resulting drift factor will be.
    
               This option was added in v2.26, because it is typical for
               systems to call **hwclock --systohc** at shutdown; with the old
               behavior this would automatically (re)calculate the drift
               factor which caused several problems:
    
               •   When using NTP with an '11 minute mode' kernel the drift
                   factor would be clobbered to near zero.
    
               •   It would not allow the use of 'cold' drift correction.
                   With most configurations using 'cold' drift will yield
                   favorable results. Cold, means when the machine is turned
                   off which can have a significant impact on the drift
                   factor.
    
               •   (Re)calculating drift factor on every shutdown delivers
                   suboptimal results. For example, if ephemeral conditions
                   cause the machine to be abnormally hot the drift factor
                   calculation would be out of range.
    
               •   Significantly increased system shutdown times (as of v2.31
                   when not using **--update-drift** the RTC is not read).
    
               Having **hwclock** calculate the drift factor is a good starting
               point, but for optimal results it will likely need to be
               adjusted by directly editing the _/etc/adjtime_ file. For most
               configurations once a machine’s optimal drift factor is
               crafted it should not need to be changed. Therefore, the old
               behavior to automatically (re)calculate drift was changed and
               now requires this option to be used. See the discussion below,
               under **The Adjust Function**.
    
               This option requires reading the Hardware Clock before setting
               it. If it cannot be read, then this option will cause the set
               functions to fail. This can happen, for example, if the
               Hardware Clock is corrupted by a power failure. In that case,
               the clock must first be set without this option. Despite it
               not working, the resulting drift correction factor would be
               invalid anyway.
    
           **-v** , **--verbose**
               Display more details about what **hwclock** is doing internally.
    


## NOTES



       **Clocks in a Linux System**
           There are two types of date-time clocks:
    
           **The Hardware Clock:** This clock is an independent hardware device,
           with its own power domain (battery, capacitor, etc), that operates
           when the machine is powered off, or even unplugged.
    
           On an ISA compatible system, this clock is specified as part of
           the ISA standard. A control program can read or set this clock
           only to a whole second, but it can also detect the edges of the 1
           second clock ticks, so the clock actually has virtually infinite
           precision.
    
           This clock is commonly called the hardware clock, the real time
           clock, the RTC, the BIOS clock, and the CMOS clock. Hardware
           Clock, in its capitalized form, was coined for use by **hwclock**. The
           Linux kernel also refers to it as the persistent clock.
    
           Some non-ISA systems have a few real time clocks with only one of
           them having its own power domain. A very low power external I2C or
           SPI clock chip might be used with a backup battery as the hardware
           clock to initialize a more functional integrated real-time clock
           which is used for most other purposes.
    
           **The System Clock:** This clock is part of the Linux kernel and is
           driven by a timer interrupt. (On an ISA machine, the timer
           interrupt is part of the ISA standard.) It has meaning only while
           Linux is running on the machine. The System Time is the number of
           seconds since 00:00:00 January 1, 1970 UTC (or more succinctly,
           the number of seconds since 1969 UTC). The System Time is not an
           integer, though. It has virtually infinite precision.
    
           The System Time is the time that matters. The Hardware Clock’s
           basic purpose is to keep time when Linux is not running so that
           the System Clock can be initialized from it at boot. Note that in
           DOS, for which ISA was designed, the Hardware Clock is the only
           real time clock.
    
           It is important that the System Time not have any discontinuities
           such as would happen if you used the date(1) program to set it
           while the system is running. You can, however, do whatever you
           want to the Hardware Clock while the system is running, and the
           next time Linux starts up, it will do so with the adjusted time
           from the Hardware Clock. Note: currently this is not possible on
           most systems because **hwclock --systohc** is called at shutdown.
    
           The Linux kernel’s timezone is set by **hwclock**. But don’t be misled
           — almost nobody cares what timezone the kernel thinks it is in.
           Instead, programs that care about the timezone (perhaps because
           they want to display a local time for you) almost always use a
           more traditional method of determining the timezone: They use the
           **TZ** environment variable or the _/etc/localtime_ file, as explained
           in the man page for tzset(3). However, some programs and fringe
           parts of the Linux kernel such as filesystems use the kernel’s
           timezone value. An example is the vfat filesystem. If the kernel
           timezone value is wrong, the vfat filesystem will report and set
           the wrong timestamps on files. Another example is the kernel’s NTP
           '11 minute mode'. If the kernel’s timezone value and/or the
           _persistent_clock_is_local_ variable are wrong, then the Hardware
           Clock will be set incorrectly by '11 minute mode'. See the
           discussion below, under **Automatic Hardware Clock Synchronization**
           **by the Kernel**.
    
           **hwclock** sets the kernel’s timezone to the value indicated by **TZ** or
           _/etc/localtime_ with the **--hctosys** or **--systz** functions.
    
           The kernel’s timezone value actually consists of two parts: 1) a
           field tz_minuteswest indicating how many minutes local time (not
           adjusted for DST) lags behind UTC, and 2) a field tz_dsttime
           indicating the type of Daylight Savings Time (DST) convention that
           is in effect in the locality at the present time. This second
           field is not used under Linux and is always zero. See also
           settimeofday(2).
    
       **Hardware Clock Access Methods**
           **hwclock** uses many different ways to get and set Hardware Clock
           values. The most normal way is to do I/O to the rtc device special
           file, which is presumed to be driven by the rtc device driver.
           Also, Linux systems using the rtc framework with udev, are capable
           of supporting multiple Hardware Clocks. This may bring about the
           need to override the default rtc device by specifying one with the
           **--rtc** option.
    
           However, this method is not always available as older systems do
           not have an rtc driver. On these systems, the method of accessing
           the Hardware Clock depends on the system hardware.
    
           On an ISA compatible system, **hwclock** can directly access the "CMOS
           memory" registers that constitute the clock, by doing I/O to Ports
           0x70 and 0x71. It does this with actual I/O instructions and
           consequently can only do it if running with superuser effective
           userid. This method may be used by specifying the **--directisa**
           option.
    
           This is a really poor method of accessing the clock, for all the
           reasons that userspace programs are generally not supposed to do
           direct I/O and disable interrupts. **hwclock** provides it for
           testing, troubleshooting, and because it may be the only method
           available on ISA systems which do not have a working rtc device
           driver.
    
       **The Adjust Function**
           The Hardware Clock is usually not very accurate. However, much of
           its inaccuracy is completely predictable - it gains or loses the
           same amount of time every day. This is called systematic drift.
           **hwclock** 's **--adjust** function lets you apply systematic drift
           corrections to the Hardware Clock.
    
           It works like this: **hwclock** keeps a file, _/etc/adjtime_ , that keeps
           some historical information. This is called the adjtime file.
    
           Suppose you start with no adjtime file. You issue a **hwclock --set**
           command to set the Hardware Clock to the true current time.
           **hwclock** creates the adjtime file and records in it the current
           time as the last time the clock was calibrated. Five days later,
           the clock has gained 10 seconds, so you issue a **hwclock --set**
           **--update-drift** command to set it back 10 seconds. **hwclock** updates
           the adjtime file to show the current time as the last time the
           clock was calibrated, and records 2 seconds per day as the
           systematic drift rate. 24 hours go by, and then you issue a
           **hwclock --adjust** command. **hwclock** consults the adjtime file and
           sees that the clock gains 2 seconds per day when left alone and
           that it has been left alone for exactly one day. So it subtracts 2
           seconds from the Hardware Clock. It then records the current time
           as the last time the clock was adjusted. Another 24 hours go by
           and you issue another **hwclock --adjust**. **hwclock** does the same
           thing: subtracts 2 seconds and updates the adjtime file with the
           current time as the last time the clock was adjusted.
    
           When you use the **--update-drift** option with **--set** or **--systohc** ,
           the systematic drift rate is (re)calculated by comparing the fully
           drift corrected current Hardware Clock time with the new set time,
           from that it derives the 24 hour drift rate based on the last
           calibrated timestamp from the adjtime file. This updated drift
           factor is then saved in _/etc/adjtime_.
    
           A small amount of error creeps in when the Hardware Clock is set,
           so **--adjust** refrains from making any adjustment that is less than
           1 second. Later on, when you request an adjustment again, the
           accumulated drift will be more than 1 second and **--adjust** will
           make the adjustment including any fractional amount.
    
           **hwclock --hctosys** also uses the adjtime file data to compensate
           the value read from the Hardware Clock before using it to set the
           System Clock. It does not share the 1 second limitation of
           **--adjust** , and will correct sub-second drift values immediately. It
           does not change the Hardware Clock time nor the adjtime file. This
           may eliminate the need to use **--adjust** , unless something else on
           the system needs the Hardware Clock to be compensated.
    
       **The Adjtime File**
           While named for its historical purpose of controlling adjustments
           only, it actually contains other information used by **hwclock** from
           one invocation to the next.
    
           The format of the adjtime file is, in ASCII:
    
           Line 1: Three numbers, separated by blanks: 1) the systematic
           drift rate in seconds per day, floating point decimal; 2) the
           resulting number of seconds since 1969 UTC of most recent
           adjustment or calibration, decimal integer; 3) zero (for
           compatibility with **clock**(8)) as a floating point decimal.
    
           Line 2: One number: the resulting number of seconds since 1969 UTC
           of most recent calibration. Zero if there has been no calibration
           yet or it is known that any previous calibration is moot (for
           example, because the Hardware Clock has been found, since that
           calibration, not to contain a valid time). This is a decimal
           integer.
    
           Line 3: "UTC" or "LOCAL". Tells whether the Hardware Clock is set
           to Coordinated Universal Time or local time. You can always
           override this value with options on the **hwclock** command line.
    
           You can use an adjtime file that was previously used with the
           **clock**(8) program with **hwclock**.
    
       **Automatic Hardware Clock Synchronization by the Kernel**
           You should be aware of another way that the Hardware Clock is kept
           synchronized in some systems. The Linux kernel has a mode wherein
           it copies the System Time to the Hardware Clock every 11 minutes.
           This mode is a compile time option, so not all kernels will have
           this capability. This is a good mode to use when you are using
           something sophisticated like NTP to keep your System Clock
           synchronized. (NTP is a way to keep your System Time synchronized
           either to a time server somewhere on the network or to a radio
           clock hooked up to your system. See RFC 1305.)
    
           If the kernel is compiled with the '11 minute mode' option it will
           be active when the kernel’s clock discipline is in a synchronized
           state. When in this state, bit 6 (the bit that is set in the mask
           0x0040) of the kernel’s _time_status_ variable is unset. This value
           is output as the 'status' line of the **adjtimex --print** or **ntptime**
           commands.
    
           It takes an outside influence, like the NTP daemon to put the
           kernel’s clock discipline into a synchronized state, and therefore
           turn on '11 minute mode'. It can be turned off by running anything
           that sets the System Clock the old fashioned way, including
           **hwclock --hctosys**. However, if the NTP daemon is still running, it
           will turn '11 minute mode' back on again the next time it
           synchronizes the System Clock.
    
           If your system runs with '11 minute mode' on, it may need to use
           either **--hctosys** or **--systz** in a startup script, especially if the
           Hardware Clock is configured to use the local timescale. Unless
           the kernel is informed of what timescale the Hardware Clock is
           using, it may clobber it with the wrong one. The kernel uses UTC
           by default.
    
           The first userspace command to set the System Clock informs the
           kernel what timescale the Hardware Clock is using. This happens
           via the _persistent_clock_is_local_ kernel variable. If **--hctosys** or
           **--systz** is the first, it will set this variable according to the
           adjtime file or the appropriate command-line argument. Note that
           when using this capability and the Hardware Clock timescale
           configuration is changed, then a reboot is required to notify the
           kernel.
    
           **hwclock --adjust** should not be used with NTP '11 minute mode'.
    
       **ISA Hardware Clock Century value**
           There is some sort of standard that defines CMOS memory Byte 50 on
           an ISA machine as an indicator of what century it is. **hwclock** does
           not use or set that byte because there are some machines that
           don’t define the byte that way, and it really isn’t necessary
           anyway, since the year-of-century does a good job of implying
           which century it is.
    
           If you have a bona fide use for a CMOS century byte, contact the
           **hwclock** maintainer; an option may be appropriate.
    
           Note that this section is only relevant when you are using the
           "direct ISA" method of accessing the Hardware Clock. ACPI provides
           a standard way to access century values, when they are supported
           by the hardware.
    


## DATE-TIME CONFIGURATION



       **Keeping Time without External Synchronization**
           This discussion is based on the following conditions:
    
           •   Nothing is running that alters the date-time clocks, such as
               NTP daemon or a cron job."
    
           •   The system timezone is configured for the correct local time.
               See below, under **POSIX vs 'RIGHT'**.
    
           •   Early during startup the following are called, in this order:
               **adjtimex --tick** _value_ **--frequency** _value_ **hwclock --hctosys**
    
           •   During shutdown the following is called: **hwclock --systohc**
    
               •   Systems without **adjtimex** may use **ntptime**.
    
           Whether maintaining precision time with NTP daemon or not, it
           makes sense to configure the system to keep reasonably good
           date-time on its own.
    
           The first step in making that happen is having a clear
           understanding of the big picture. There are two completely
           separate hardware devices running at their own speed and drifting
           away from the 'correct' time at their own rates. The methods and
           software for drift correction are different for each of them.
           However, most systems are configured to exchange values between
           these two clocks at startup and shutdown. Now the individual
           device’s time keeping errors are transferred back and forth
           between each other. Attempt to configure drift correction for only
           one of them, and the other’s drift will be overlaid upon it.
    
           This problem can be avoided when configuring drift correction for
           the System Clock by simply not shutting down the machine. This,
           plus the fact that all of **hwclock** 's precision (including
           calculating drift factors) depends upon the System Clock’s rate
           being correct, means that configuration of the System Clock should
           be done first.
    
           The System Clock drift is corrected with the **adjtimex**(8) command’s
           **--tick** and **--frequency** options. These two work together: tick is
           the coarse adjustment and frequency is the fine adjustment. (For
           systems that do not have an **adjtimex** package, **ntptime -f** _ppm_ may
           be used instead.)
    
           Some Linux distributions attempt to automatically calculate the
           System Clock drift with **adjtimex** 's compare operation. Trying to
           correct one drifting clock by using another drifting clock as a
           reference is akin to a dog trying to catch its own tail. Success
           may happen eventually, but great effort and frustration will
           likely precede it. This automation may yield an improvement over
           no configuration, but expecting optimum results would be in error.
           A better choice for manual configuration would be **adjtimex** 's **--log**
           options.
    
           It may be more effective to simply track the System Clock drift
           with **sntp** , or **date -Ins** and a precision timepiece, and then
           calculate the correction manually.
    
           After setting the tick and frequency values, continue to test and
           refine the adjustments until the System Clock keeps good time. See
           adjtimex(2) for more information and the example demonstrating
           manual drift calculations.
    
           Once the System Clock is ticking smoothly, move on to the Hardware
           Clock.
    
           As a rule, cold drift will work best for most use cases. This
           should be true even for 24/7 machines whose normal downtime
           consists of a reboot. In that case the drift factor value makes
           little difference. But on the rare occasion that the machine is
           shut down for an extended period, then cold drift should yield
           better results.
    
           **Steps to calculate cold drift:**
    
           1
               **Ensure that NTP daemon will not be launched at startup.**
    
           2
               The _System Clock_ time must be correct at shutdown!
    
           3
               Shut down the system.
    
           4
               Let an extended period pass without changing the Hardware
               Clock.
    
           5
               Start the system.
    
           6
               Immediately use **hwclock** to set the correct time, adding the
               **--update-drift** option.
    
           Note: if step 6 uses **--systohc** , then the System Clock must be set
           correctly (step 6a) just before doing so.
    
           Having **hwclock** calculate the drift factor is a good starting
           point, but for optimal results it will likely need to be adjusted
           by directly editing the _/etc/adjtime_ file. Continue to test and
           refine the drift factor until the Hardware Clock is corrected
           properly at startup. To check this, first make sure that the
           System Time is correct before shutdown and then use **sntp** , or **date**
           **-Ins** and a precision timepiece, immediately after startup.
    
       **LOCAL vs UTC**
           Keeping the Hardware Clock in a local timescale causes
           inconsistent daylight saving time results:
    
           •   If Linux is running during a daylight saving time change, the
               time written to the Hardware Clock will be adjusted for the
               change.
    
           •   If Linux is NOT running during a daylight saving time change,
               the time read from the Hardware Clock will NOT be adjusted for
               the change.
    
           The Hardware Clock on an ISA compatible system keeps only a date
           and time, it has no concept of timezone nor daylight saving.
           Therefore, when **hwclock** is told that it is in local time, it
           assumes it is in the 'correct' local time and makes no adjustments
           to the time read from it.
    
           Linux handles daylight saving time changes transparently only when
           the Hardware Clock is kept in the UTC timescale. Doing so is made
           easy for system administrators as **hwclock** uses local time for its
           output and as the argument to the **--date** option.
    
           POSIX systems, like Linux, are designed to have the System Clock
           operate in the UTC timescale. The Hardware Clock’s purpose is to
           initialize the System Clock, so also keeping it in UTC makes
           sense.
    
           Linux does, however, attempt to accommodate the Hardware Clock
           being in the local timescale. This is primarily for dual-booting
           with older versions of MS Windows. From Windows 7 on, the
           RealTimeIsUniversal registry key is supposed to be working
           properly so that its Hardware Clock can be kept in UTC.
    
       **POSIX vs 'RIGHT'**
           A discussion on date-time configuration would be incomplete
           without addressing timezones, this is mostly well covered by
           tzset(3). One area that seems to have no documentation is the
           'right' directory of the Time Zone Database, sometimes called tz
           or zoneinfo.
    
           There are two separate databases in the zoneinfo system, posix and
           'right'. 'Right' (now named zoneinfo-leaps) includes leap seconds
           and posix does not. To use the 'right' database the System Clock
           must be set to (UTC + leap seconds), which is equivalent to (TAI -
           10). This allows calculating the exact number of seconds between
           two dates that cross a leap second epoch. The System Clock is then
           converted to the correct civil time, including UTC, by using the
           'right' timezone files which subtract the leap seconds. Note: this
           configuration is considered experimental and is known to have
           issues.
    
           To configure a system to use a particular database all of the
           files located in its directory must be copied to the root of
           _/usr/share/zoneinfo_. Files are never used directly from the posix
           or 'right' subdirectories, e.g., TZ='_right/Europe/Dublin_ '. This
           habit was becoming so common that the upstream zoneinfo project
           restructured the system’s file tree by moving the posix and
           'right' subdirectories out of the zoneinfo directory and into
           sibling directories:
    
           _/usr/share/zoneinfo_ , _/usr/share/zoneinfo-posix_ ,
           _/usr/share/zoneinfo-leaps_
    
           Unfortunately, some Linux distributions are changing it back to
           the old tree structure in their packages. So the problem of system
           administrators reaching into the 'right' subdirectory persists.
           This causes the system timezone to be configured to include leap
           seconds while the zoneinfo database is still configured to exclude
           them. Then when an application such as a World Clock needs the
           South_Pole timezone file; or an email MTA, or **hwclock** needs the
           UTC timezone file; they fetch it from the root of
           _/usr/share/zoneinfo_ , because that is what they are supposed to
           do. Those files exclude leap seconds, but the System Clock now
           includes them, causing an incorrect time conversion.
    
           Attempting to mix and match files from these separate databases
           will not work, because they each require the System Clock to use a
           different timescale. The zoneinfo database must be configured to
           use either posix or 'right', as described above, or by assigning a
           database path to the _TZDIR_ environment variable.
    


## EXIT STATUS



           One of the following exit values will be returned:
    
           **EXIT_SUCCESS**('0' on POSIX systems)
               Successful program execution.
    
           **EXIT_FAILURE**('1' on POSIX systems)
               The operation failed or the command syntax was not valid.
    


## ENVIRONMENT



           **TZ**
               If this variable is set its value takes precedence over the
               system configured timezone.
    
           **TZDIR**
               If this variable is set its value takes precedence over the
               system configured timezone database directory path.
    


## FILES



           _/etc/adjtime_
               The configuration and state file for **hwclock**. See also
               adjtime_config(5).
    
           _/etc/localtime_
               The system timezone file.
    
           _/usr/share/zoneinfo/_
               The system timezone database directory.
    
           Device files **hwclock** may try for Hardware Clock access: _/dev/rtc_
           _/dev/rtc0 /dev/misc/rtc /dev/efirtc /dev/misc/efirtc_
    


## AVAILABILITY



           The **hwclock** command is part of the util-linux package which can be
           downloaded from Linux Kernel Archive
           <https://www.kernel.org/pub/linux/utils/util-linux/>. This page is
           part of the _util-linux_ (a random collection of Linux utilities)
           project. Information about the project can be found at 
           ⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩. If you have a
           bug report for this manual page, send it to
           util-linux@vger.kernel.org. This page was obtained from the
           project's upstream Git repository
           ⟨git://git.kernel.org/pub/scm/utils/util-linux/util-linux.git⟩ on
           2025-08-11. (At that time, the date of the most recent commit that
           was found in the repository was 2025-08-05.) If you discover any
           rendering problems in this HTML version of the page, or you
           believe there is a better or more up-to-date source for the page,
           or you have corrections or improvements to the information in this
           COLOPHON (which is _not_ part of the original manual page), send a
           mail to man-pages@man7.org
    
    util-linux 2.42-start-521-ec46  2025-08-09                     _HWCLOCK_(8)
    


* * *