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 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.

       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 sys-
              tematic 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 avail-
              able through the Linux kernel RTC driver.

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

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

                  hwclock --setepoch --epoch=1952

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

              This epoch value is used whenever  hwclock  reads  or  sets  the
              Hardware Clock on an Alpha machine.  For ISA machines the kernel
              uses the fixed Hardware Clock epoch of 1900.

              Predict what the Hardware Clock will read in  the  future  based
              upon  the time given by the --date option and the information in
              /etc/adjtime.  This is useful, for example, to account for drift
              when   setting   a   Hardware  Clock  wakeup  (aka  alarm).  See

              Do not use this function if the Hardware Clock is being modified
              by  anything  other  than the current operating system's hwclock
              command, such as '11 minute mode' or from  dual-booting  another

       -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 any-
              thing 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 discus-
              sion 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 lo-
              cal  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 ob-
              solete 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 configura-
              tion is changed then a reboot is required to inform the  kernel.
              See  the  discussion  below, under Automatic Hardware Clock Syn-
              chronization 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 func-
              tion.  A third method is to delete the /etc/adjtime  file.   Hw-
              clock will then default to using the UTC timescale for the Hard-
              ware Clock.  If the Hardware Clock is ticking local time it will
              need to be defined in the file.  This can be done by calling hw-
              clock --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:

              o Corrects  the  System  Clock timescale to UTC as needed.  Only
                instead of accomplishing this by setting the System Clock, hw-
                clock simply informs the kernel and it handles the change.

              o Sets the kernel's NTP '11 minute mode' timescale.

              o 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  re-
              quired 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 Hard-
              ware 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 argu-
              ment's value and when the enter key is pressed.  Fractional sec-
              onds  are  silently  dropped.   This option is capable of under-
              standing many time and date formats, but the previous parameters
              should be observed.

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

              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 op-

              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 sys-
              tems to call hwclock --systohc at shutdown; with the old  behav-
              iour  this  would  automatically  (re)calculate the drift factor
              which caused several problems:

              o When using NTP with an '11 minute mode' kernel the drift  fac-
                tor would be clobbered to near zero.

              o 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.

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

              o 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 ad-
              justed by directly editing the /etc/adjtime file.  For most con-
              figurations  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 Ad-
              just 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 Hard-
              ware 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

       -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  virtu-
       ally infinite precision.

       The  System  Time is the time that matters.  The Hardware Clock's basic
       purpose is to keep time when Linux is not running so  that  the  System
       Clock  can be initialized from it at boot.  Note that in DOS, for which
       ISA was designed, the Hardware Clock is the only real time clock.

       It is important that the System Time not have any discontinuities  such
       as  would  happen  if  you used the date(1) program to set it while the
       system is running.  You can, however, do whatever you want to the Hard-
       ware  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  vari-
       able  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

       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  local-
       ity 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 mem-
       ory" 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 rea-
       sons that userspace programs are generally not supposed  to  do  direct
       I/O  and  disable  interrupts.   hwclock provides it for testing, trou-
       bleshooting, 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 com-
       mand  to set the Hardware Clock to the true current time.  hwclock cre-
       ates 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 cur-
       rent time as the last time the clock was calibrated, and records 2 sec-
       onds  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  time-
       stamp  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 sec-
       ond.  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 invoca-
       tion 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 num-
       ber of seconds since 1969 UTC of most recent adjustment or calibration,
       decimal integer; 3) zero (for compatibility with clock(8)) as a 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 con-
       tain 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 syn-
       chronized 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  capa-
       bility.   This  is  a good mode to use when you are using something so-
       phisticated like NTP to keep your System Clock synchronized. (NTP is  a
       way to keep your System Time synchronized either to a time server some-
       where 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 capa-
       bility 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:

       o Nothing is running that alters the date-time clocks, such as NTP dae-
         mon or a cron job."

       o The  system  timezone  is configured for the correct local time.  See
         below, under POSIX vs 'RIGHT'.

       o Early during startup the following are called, in this order:
         adjtimex --tick value --frequency value
         hwclock --hctosys

       o 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

       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  fac-
       tors)  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 in-

       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  automa-
       tion  may yield an improvement over no configuration, but expecting op-
       timum results would be in error.  A better choice for manual configura-
       tion 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 re-
       fine the adjustments until the  System  Clock  keeps  good  time.   See
       adjtimex(8)  for  more information and the example demonstrating manual
       drift calculations.

       Once the System Clock is ticking smoothly,  move  on  to  the  Hardware

       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  re-
       boot.   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  cor-
       rectly (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, im-
       mediately after startup.

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

       o If Linux is running during a daylight saving time  change,  the  time
         written to the Hardware Clock will be adjusted for the change.

       o If  Linux  is  NOT  running during a daylight saving time change, the
         time read from the Hardware  Clock  will  NOT  be  adjusted  for  the

       The  Hardware  Clock  on an ISA compatible system keeps only a date and
       time, it has no concept of timezone  nor  daylight  saving.  Therefore,
       when  hwclock is told that it is in local time, it assumes it is in the
       'correct' local time and makes no adjustments to the time read from it.

       Linux handles daylight saving time changes transparently only when  the
       Hardware  Clock is kept in the UTC timescale. Doing so is made easy for
       system administrators as hwclock uses local time for its output and  as
       the argument to the --date option.

       POSIX  systems, like Linux, are designed to have the System Clock oper-
       ate 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 ver-
       sions  of  MS  Windows. From Windows 7 on, the RealTimeIsUniversal reg-
       istry 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 ad-
       dressing 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 lo-
       cated   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 administra-
       tors 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 ap-
       plication  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 sys-
              tem configured timezone.

       TZDIR  If this variable is set its value takes precedence over the sys-
              tem 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(1),

       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

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

util-linux                         July 2017                        HWCLOCK(8)
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