hwclock(8) — Linux manual page


HWCLOCK(8)                  System Administration                 HWCLOCK(8)

NAME         top

       hwclock - time clocks utility

SYNOPSIS         top

       hwclock [function] [option...]

DESCRIPTION         top

       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         top

       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

              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 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

              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
              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

       -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

              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

              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         top

              Override the default /etc/adjtime file path.

              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.

              This option can be used to overwrite the internally used delay
              when setting the 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.

              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.

              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:
              For IA-64:

       -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

              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.

              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

              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         top

   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

       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

       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

       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

       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

       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


   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

       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

       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

       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.

       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

       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.

       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


       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         top

       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         top

       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         top

              The configuration and state file for hwclock.

              The system timezone file.

              The system timezone database directory.

       Device files hwclock may try for Hardware Clock access:

SEE ALSO         top

       date(1), adjtimex(8), gettimeofday(2), settimeofday(2), crontab(1p),

AUTHORS         top

       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         top

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

COLOPHON         top

       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
       2020-09-18.  (At that time, the date of the most recent commit that
       was found in the repository was 2020-09-15.)  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

util-linux                        July 2017                       HWCLOCK(8)

Pages that refer to this page: timedatectl(1)adjtimex(2)clock_adjtime(2)clock_getres(2)clock_gettime(2)clock_settime(2)gettimeofday(2)settimeofday(2)ntp_adjtime(3)rtc(4)adjtime(5)adjtime_config(5)time(7)rtcwake(8)systemd-timedated(8)systemd-timedated.service(8)systemd-timesyncd(8)systemd-timesyncd.service(8)