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 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
              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

              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

              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

              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.

              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

              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

       -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 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.

              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 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

              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

              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

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

       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

       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

   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

       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

       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.


   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

       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

       • 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

       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 directories:


       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), tzset(3)

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

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
       2021-03-21.  (At that time, the date of the most recent commit
       that was found in the repository was 2021-03-19.)  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                      July 2017                     HWCLOCK(8)

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