HWCLOCK(8)                    System Administration                   HWCLOCK(8)

       hwclock - time clocks utility

       hwclock [function] [option...]

       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

       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.

       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.

              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

              Showing the Hardware Clock time is the default when no function is

              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 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.

              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.

              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

              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 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.

   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

       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

       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

       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

   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

       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

       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.

   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

       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.

       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.

       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.

       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.

       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.

              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:

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

       Written by Bryan Henderson, September 1996 (,
       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.

       The hwclock command is part of the util-linux package and is available

util-linux                          July 2017                         HWCLOCK(8)