Provided by: util-linux_2.37.2-4ubuntu3.4_amd64 bug


       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 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


       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

       --getepoch; --setepoch
           These functions are for Alpha machines only, and are only available through the Linux
           kernel RTC driver.

           They are used to read and set the kernel’s Hardware Clock epoch value. Epoch is the
           number of years into AD to which a zero year value in the Hardware Clock refers. For
           example, if the machine’s BIOS sets the year counter in the Hardware Clock to contain
           the number of full years since 1952, then the kernel’s Hardware Clock epoch value must
           be 1952.

           The --setepoch function requires using the --epoch option to specify the year. For

           hwclock --setepoch --epoch=1952

           The RTC driver attempts to guess the correct epoch value, so setting it may not be

           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; --get
           Read the Hardware Clock and print its time to standard output in the ISO 8601 format.
           The time shown is always in local time, even if you keep your Hardware Clock in UTC.
           See the --localtime option.

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

           The --get function also applies drift correction to the time read, based upon the
           information in /etc/adjtime. Do not use this function if the Hardware Clock is being
           modified by anything other than the current operating system’s hwclock command, such
           as '11 minute mode' or from dual-booting another OS.

       -s, --hctosys
           Set the System Clock from the Hardware Clock. The time read from the Hardware Clock is
           compensated to account for systematic drift before using it to set the System Clock.
           See the discussion below, under The Adjust Function.

           The System Clock must be kept in the UTC timescale for date-time applications to work
           correctly in conjunction with the timezone configured for the system. If the Hardware
           Clock is kept in local time then the time read from it must be shifted to the UTC
           timescale before using it to set the System Clock. The --hctosys function does this
           based upon the information in the /etc/adjtime file or the command line arguments
           --localtime and --utc. Note: no daylight saving adjustment is made. See the discussion
           below, under LOCAL vs UTC.

           The kernel also keeps a timezone value, the --hctosys function sets it to the timezone
           configured for the system. The system timezone is configured by the TZ environment
           variable or the /etc/localtime file, as tzset(3) would interpret them. The obsolete
           tz_dsttime field of the kernel’s timezone value is set to zero. (For details on what
           this field used to mean, see settimeofday(2).)

           When used in a startup script, making the --hctosys function the first caller of
           settimeofday(2) from boot, it will set the NTP '11 minute mode' timescale via the
           persistent_clock_is_local kernel variable. If the Hardware Clock’s timescale
           configuration is changed then a reboot is required to inform the kernel. See the
           discussion below, under Automatic Hardware Clock Synchronization by the Kernel.

           This is a good function to use in one of the system startup scripts before the file
           systems are mounted read/write.

           This function should never be used on a running system. Jumping system time will cause
           problems, such as corrupted filesystem timestamps. Also, if something has changed the
           Hardware Clock, like NTP’s '11 minute mode', then --hctosys will set the time
           incorrectly by including drift compensation.

           Drift compensation can be inhibited by setting the drift factor in /etc/adjtime to
           zero. This setting will be persistent as long as the --update-drift option is not used
           with --systohc at shutdown (or anywhere else). Another way to inhibit this is by using
           the --noadjfile option when calling the --hctosys function. A third method is to
           delete the /etc/adjtime file. Hwclock will then default to using the UTC timescale for
           the Hardware Clock. If the Hardware Clock is ticking local time it will need to be
           defined in the file. This can be done by calling hwclock --localtime --adjust; when
           the file is not present this command will not actually adjust the Clock, but it will
           create the file with local time configured, and a drift factor of zero.

           A condition under which inhibiting hwclock's drift correction may be desired is when
           dual-booting multiple operating systems. If while this instance of Linux is stopped,
           another OS changes the Hardware Clock’s value, then when this instance is started
           again the drift correction applied will be incorrect.

           For hwclock's drift correction to work properly it is imperative that nothing changes
           the Hardware Clock while its Linux instance is not running.

           Set 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

       -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

           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: /dev/rtc0, /dev/rtc, /dev/misc/rtc. For IA-64: /dev/efirtc

       -l, --localtime; -u, --utc
           Indicate which timescale the Hardware Clock is set to.

           The Hardware Clock may be configured to use either the UTC or the local timescale, but
           nothing in the clock itself says which alternative is being used. The --localtime or
           --utc options give this information to the hwclock command. If you specify the wrong
           one (or specify neither and take a wrong default), both setting and reading the
           Hardware Clock will be incorrect.

           If you specify neither --utc nor --localtime then the one last given with a set
           function (--set, --systohc, or --adjust), as recorded in /etc/adjtime, will be used.
           If the adjtime file doesn’t exist, the default is UTC.

           Note: daylight saving time changes may be inconsistent when the Hardware Clock is kept
           in local time. See the discussion below, under LOCAL vs UTC.

           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.

           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 behavior this would automatically (re)calculate
           the drift factor which caused several problems:

           •   When using NTP with an '11 minute mode' kernel the drift factor would be clobbered
               to near zero.

           •   It would not allow the use of 'cold' drift correction. With most configurations
               using 'cold' drift will yield favorable results. Cold, means when the machine is
               turned off which can have a significant impact on the drift factor.

           •   (Re)calculating drift factor on every shutdown delivers suboptimal results. For
               example, if ephemeral conditions cause the machine to be abnormally hot the drift
               factor calculation would be out of range.

           •   Significantly increased system shutdown times (as of v2.31 when not using
               --update-drift the RTC is not read).

       Having hwclock calculate the drift factor is a good starting point, but for optimal
       results it will likely need to be adjusted by directly editing the /etc/adjtime file. For
       most configurations once a machine’s optimal drift factor is crafted it should not need to
       be changed. Therefore, the old behavior to automatically (re)calculate drift was changed
       and now requires this option to be used. See the discussion below, under The Adjust

       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

       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

       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

       On an ISA compatible system, hwclock can directly access the "CMOS memory" registers that
       constitute the clock, by doing I/O to Ports 0x70 and 0x71. It does this with actual I/O
       instructions and consequently can only do it if running with superuser effective userid.
       This method may be used by specifying the --directisa option.

       This is a really poor method of accessing the clock, for all the reasons that userspace
       programs are generally not supposed to do direct I/O and disable interrupts. hwclock
       provides it for testing, troubleshooting, and because it may be the only method available
       on ISA systems which do not have a working rtc device driver.

   The Adjust Function
       The Hardware Clock is usually not very accurate. However, much of its inaccuracy is
       completely predictable - it gains or loses the same amount of time every day. This is
       called systematic drift. hwclock's --adjust function lets you apply systematic drift
       corrections to the Hardware Clock.

       It works like this: hwclock keeps a file, /etc/adjtime, that keeps some historical
       information. This is called the adjtime file.

       Suppose you start with no adjtime file. You issue a hwclock --set command to set the
       Hardware Clock to the true current time. hwclock creates the adjtime file and records in
       it the current time as the last time the clock was calibrated. Five days later, the clock
       has gained 10 seconds, so you issue a hwclock --set --update-drift command to set it back
       10 seconds. hwclock updates the adjtime file to show the current time as the last time the
       clock was calibrated, and records 2 seconds per day as the systematic drift rate. 24 hours
       go by, and then you issue a hwclock --adjust command. hwclock consults the adjtime file
       and sees that the clock gains 2 seconds per day when left alone and that it has been left
       alone for exactly one day. So it subtracts 2 seconds from the Hardware Clock. It then
       records the current time as the last time the clock was adjusted. Another 24 hours go by
       and you issue another hwclock --adjust. hwclock does the same thing: subtracts 2 seconds
       and updates the adjtime file with the current time as the last time the clock was

       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

       You can use an adjtime file that was previously used with the clock(8) program with

   Automatic Hardware Clock Synchronization by the Kernel
       You should be aware of another way that the Hardware Clock is kept synchronized in some
       systems. The Linux kernel has a mode wherein it copies the System Time to the Hardware
       Clock every 11 minutes. This mode is a compile time option, so not all kernels will have
       this capability. This is a good mode to use when you are using something sophisticated
       like NTP to keep your System Clock synchronized. (NTP is a way to keep your System Time
       synchronized either to a time server somewhere on the network or to a radio clock hooked
       up to your system. See RFC 1305.)

       If the kernel is compiled with the '11 minute mode' option it will be active when the
       kernel’s clock discipline is in a synchronized state. When in this state, bit 6 (the bit
       that is set in the mask 0x0040) of the kernel’s time_status variable is unset. This value
       is output as the 'status' line of the adjtimex --print or ntptime commands.

       It takes an outside influence, like the NTP daemon to put the kernel’s clock discipline
       into a synchronized state, and therefore turn on '11 minute mode'. It can be turned off by
       running anything that sets the System Clock the old fashioned way, including hwclock
       --hctosys. However, if the NTP daemon is still running, it will turn '11 minute mode' back
       on again the next time it synchronizes the System Clock.

       If your system runs with '11 minute mode' on, it may need to use either --hctosys or
       --systz in a startup script, especially if the Hardware Clock is configured to use the
       local timescale. Unless the kernel is informed of what timescale the Hardware Clock is
       using, it may clobber it with the wrong one. The kernel uses UTC by default.

       The first userspace command to set the System Clock informs the kernel what timescale the
       Hardware Clock is using. This happens via the persistent_clock_is_local kernel variable.
       If --hctosys or --systz is the first, it will set this variable according to the adjtime
       file or the appropriate command-line argument. Note that when using this capability and
       the Hardware Clock timescale configuration is changed, then a reboot is required to notify
       the kernel.

       hwclock --adjust should not be used with NTP '11 minute mode'.

   ISA Hardware Clock Century value
       There is some sort of standard that defines CMOS memory Byte 50 on an ISA machine as an
       indicator of what century it is. hwclock does not use or set that byte because there are
       some machines that don’t define the byte that way, and it really isn’t necessary anyway,
       since the year-of-century does a good job of implying which century it is.

       If you have a bona fide use for a CMOS century byte, contact the hwclock maintainer; an
       option may be appropriate.

       Note that this section is only relevant when you are using the "direct ISA" method of
       accessing the Hardware Clock. ACPI provides a standard way to access century values, when
       they are supported by the hardware.


   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

       •   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 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:

           Ensure that NTP daemon will not be launched at startup.

           The System Clock time must be correct at shutdown!

           Shut down the system.

           Let an extended period pass without changing the Hardware Clock.

           Start the system.

           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

       Keeping the Hardware Clock in a local timescale causes inconsistent daylight saving time

       •   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:

       /usr/share/zoneinfo, /usr/share/zoneinfo-posix, /usr/share/zoneinfo-leaps

       Unfortunately, some Linux distributions are changing it back to the old tree structure in
       their packages. So the problem of system administrators reaching into the 'right'
       subdirectory persists. This causes the system timezone to be configured to include leap
       seconds while the zoneinfo database is still configured to exclude them. Then when an
       application such as a World Clock needs the South_Pole timezone file; or an email MTA, or
       hwclock needs the UTC timezone file; they fetch it from the root of /usr/share/zoneinfo ,
       because that is what they are supposed to do. Those files exclude leap seconds, but the
       System Clock now includes them, causing an incorrect time conversion.

       Attempting to mix and match files from these separate databases will not work, because
       they each require the System Clock to use a different timescale. The zoneinfo database
       must be configured to use either posix or 'right', as described above, or by assigning a
       database path to the TZDIR environment variable.


       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.


           If this variable is set its value takes precedence over the system configured

           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: /dev/rtc0 /dev/rtc /dev/misc/rtc
       /dev/efirtc /dev/misc/efirtc


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


       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.


       For bug reports, use the issue tracker at


       The hwclock command is part of the util-linux package which can be downloaded from Linux
       Kernel Archive <>.