hwclock

HWCLOCK(8)                   System Administration                  HWCLOCK(8)



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.

       --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.

       -V, --version
              Display version information and exit.

       -h, --help
              Display help text 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 allows to overwrite internally used delay when set
              clock time. The default is 0.5 (500ms) for rtc_cmos, for another
              RTC types the delay is 0. If RTC type is impossible to determine
              (from sysfs) then it defaults also to 0.5 to be backwardly
              compatible.

              The 500ms default is based on commonly used MC146818A-compatible
              (x86) hardware clock. This Hardware Clock can only be set to any
              integer time plus one half second.  The integer time is required
              because there is no interface to set or get a fractional second.
              The additional half second delay 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 same cases another 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/rtc0
                  /dev/rtc
                  /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
              behaviour 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 decimal integer.

       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(8) 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.

       /etc/localtime
              The system timezone file.

       /usr/share/zoneinfo/
              The system timezone database directory.

       Device files hwclock may try for Hardware Clock access:
       /dev/rtc0
       /dev/rtc
       /dev/misc/rtc
       /dev/efirtc
       /dev/misc/efirtc

SEE ALSO
       date(1), adjtimex(8), gettimeofday(2), settimeofday(2), crontab(1),
       tzset(3)

AUTHORS
       Written by Bryan Henderson, September 1996 (bryanh@giraffe-data.com),
       based on work done on the clock(8) program by Charles Hedrick, Rob
       Hooft, and Harald Koenig.  See the source code for complete history and
       credits.

AVAILABILITY
       The hwclock command is part of the util-linux package and is available
       from https://www.kernel.org/pub/linux/utils/util-linux/.



util-linux                         July 2017                        HWCLOCK(8)