Provided by: util-linux_2.32-0.1ubuntu2_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 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

                  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

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

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

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

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

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

                  hwclock --set --date='16:45'

                  hwclock --predict --date='2525-08-14 07:11:05'

              The argument must be in local time, even if you keep your Hardware  Clock  in  UTC.
              See  the  --localtime  option.   Therefore,  the  argument  should  not include any
              timezone information.  It also should not be a relative  time  like  "+5  minutes",
              because  hwclock's  precision depends upon correlation between the argument's value
              and when the enter key is pressed.  Fractional seconds are silently dropped.   This
              option  is  capable  of  understanding many time and date formats, but the previous
              parameters should be observed.

       -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 ntpd 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

       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 decimal integer.

       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

   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 ntpd(1), 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 ntpd(1) or a cron job.

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

       · 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 ntpd(1) 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(8)  for  more  information  and  the
       example demonstrating manual drift calculations.

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

       As  a  rule,  cold  drift will work best for most use cases.  This should be true even for
       24/7 machines whose normal downtime consists of a reboot.  In that case the  drift  factor
       value makes little difference.  But on the rare occasion that the machine is shut down for
       an extended period, then cold drift should yield better results.

       Steps to calculate cold drift:

       1 Ensure that ntpd(1) 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

       · 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

       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(1), 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.


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