Provided by: util-linux_2.27.1-6ubuntu3.10_amd64 bug


       hwclock - read or set the hardware clock (RTC)


       hwclock [function] [option...]


       hwclock  is  a  tool for accessing the Hardware Clock.  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);  compare  the System and Hardware Clocks; 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.

              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.

       -c, --compare
              Periodically  compare  the  Hardware  Clock  to  the  System  Time  and  output the
              difference every 10 seconds.  This will also print the frequency offset and tick.

              These functions are for Alpha machines only.

              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
              you are using the convention that the year counter in your Hardware Clock  contains
              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.

              This epoch value is used whenever hwclock reads or sets the Hardware Clock.

              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 the time on standard output.  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 (re)calculate the drift

              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.  When the --update-drift option is given, then also (re)calculate the
              drift factor.

       -V, --version
              Display version information and exit.

       -h, --help
              Display help text and exit.


              Override the default /etc/adjtime file path.

              Indicate that the Hardware Clock is incapable of storing years  outside  the  range
              1994-1999.  There is a problem in some BIOSes (almost all Award BIOSes made between
              4/26/94 and 5/31/95) wherein they are unable to deal with years after 1999.  If one
              attempts  to set the year-of-century value to something less than 94 (or 95 in some
              cases), the value that actually gets set is 94 (or 95).  Thus, if you have  one  of
              these  machines, hwclock cannot set the year after 1999 and cannot use the value of
              the clock as the true time in the normal way.

              To compensate for this (without your getting a BIOS update, which would  definitely
              be  preferable),  always  use  --badyear  if  you have one of these machines.  When
              hwclock knows it's working with a brain-damaged clock, it ignores the year part  of
              the  Hardware  Clock  value  and  instead tries to guess the year based on the last
              calibrated date in the adjtime file, by assuming that date is within the past year.
              For  this  to work, you had better do a hwclock --set or hwclock --systohc at least
              once a year!

              Though hwclock ignores the year value when it reads the Hardware Clock, it sets the
              year  value  when  it  sets  the  clock.   It sets it to 1995, 1996, 1997, or 1998,
              whichever one has the same position in the leap year cycle as the true year.   That
              way, the Hardware Clock inserts leap days where they belong.  Again, if you let the
              Hardware Clock run for more than a year without setting it, this  scheme  could  be
              defeated and you could end up losing a day.

              You  need this option if you specify the --set or --predict functions, otherwise it
              is ignored.  It specifies the time to which to set the Hardware Clock, or the  time
              for  which to predict the Hardware Clock reading.  The value of this option is used
              as an argument to the date(1) program's --date option.  For example:

                  hwclock --set --date='2011-08-14 16:45:05'

              The argument must be in local time, even if you keep your Hardware  Clock  in  UTC.
              See  the  --localtime  option.   The  argument must 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.

       -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 including x86, and x86_64;
              and Alpha (which has a similar Hardware Clock interface).  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,
              which  it  assumes  to  be driven by the 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.

       -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:

       -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, i.e., the Clocks  or  adjtime  file.
              This  is  useful,  especially  in  conjunction  with --debug, in learning about the
              internal operations of hwclock.

              Update the Hardware Clock's drift factor in /etc/adjtime.  It is used with --set or
              --systohc, otherwise it is ignored.

              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.

              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.


       --arc  This  option  is  equivalent to --epoch=1980 and is used to specify the most common
              epoch on Alphas with an ARC console (although Ruffians have an epoch of 1900).

              Specifies the year which is the beginning of the Hardware Clock's  epoch,  that  is
              the  number  of  years  into  AD to which a zero value in the Hardware Clock's year
              counter refers.  It is used together with the --setepoch option to set the kernel's
              idea of the epoch of the Hardware Clock.

              For example, on a Digital Unix machine:

                  hwclock --setepoch --epoch=1952

              These two options specify what kind of Alpha machine you have.  They are invalid if
              you do not have an Alpha and are usually unnecessary if you do; hwclock  should  be
              able to determine what it is running on when /proc is mounted.

              The  --jensen  option is used for Jensen models; --funky-toy means that the machine
              requires the UF bit instead of the UIP bit in the Hardware Clock to detect  a  time
              transition.   The  "toy"  in the option name refers to the Time Of Year facility of
              the machine.

       --srm  This option is equivalent to --epoch=1900 and is used to specify  the  most  common
              epoch on Alphas with an SRM console.


   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 compatible and Alpha systems which do not have a working rtc device driver.

       In the case of a Jensen  Alpha,  there  is  no  way  for  hwclock  to  execute  those  I/O
       instructions,  and  so  it  uses instead the /dev/port device special file, which provides
       almost as low-level an interface to the I/O subsystem.

       On an m68k system, hwclock can access the clock with the console driver,  via  the  device
       special file /dev/tty1.

   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.


       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