Provided by: util-linux_2.31.1-0.4ubuntu3.7_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
              Display a lot of information about what hwclock is doing internally.  Some  of  its
              functions  are  complex  and  this  output  can help you understand how the program

              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
              (--debug 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.


   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