hwclock

HWCLOCK(8)                   System Administration                  HWCLOCK(8)

NAME
       hwclock - time clocks utility

SYNOPSIS
       hwclock [function] [option...]

DESCRIPTION
       hwclock is an administration tool for the time clocks.  It can: display
       the Hardware Clock time; set the Hardware Clock to  a  specified  time;
       set the Hardware Clock from the System Clock; set the System Clock from
       the Hardware Clock; compensate for Hardware Clock  drift;  correct  the
       System  Clock  timescale; set the kernel's timezone, NTP timescale, and
       epoch (Alpha only); and predict future Hardware Clock values  based  on
       its drift rate.

       Since  v2.26  important changes were made to the --hctosys function and
       the --directisa option, and a new option --update-drift was added.  See
       their respective descriptions below.

FUNCTIONS
       The  following  functions are mutually exclusive, only one can be given
       at a time.  If none is given, the default is --show.

       -a, --adjust
              Add or subtract time from the Hardware Clock to account for sys-
              tematic drift since the last time the clock was set or adjusted.
              See the discussion below, under The Adjust Function.

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

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

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

                  hwclock --setepoch --epoch=1952

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

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

       --predict
              Predict what the Hardware Clock will read in  the  future  based
              upon  the time given by the --date option and the information in
              /etc/adjtime.  This is useful, for example, to account for drift
              when   setting   a   Hardware  Clock  wakeup  (aka  alarm).  See
              rtcwake(8).

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

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

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

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

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

              The System Clock must be kept in the UTC timescale for date-time
              applications  to work correctly in conjunction with the timezone
              configured for the system.  If the Hardware  Clock  is  kept  in
              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 configura-
              tion is changed then a reboot is required to inform the  kernel.
              See  the  discussion  below, under Automatic Hardware Clock Syn-
              chronization by the Kernel.

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

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

              Drift  compensation can be inhibited by setting the drift factor
              in /etc/adjtime to zero.  This setting  will  be  persistent  as
              long  as the --update-drift option is not used with --systohc at
              shutdown (or anywhere else).  Another way to inhibit this is  by
              using  the  --noadjfile  option when calling the --hctosys func-
              tion.  A third  method  is  to  delete  the  /etc/adjtime  file.
              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 call-
              ing 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.

       --systz
              This is an alternate to the --hctosys  function  that  does  not
              read  the  Hardware Clock nor set the System Clock; consequently
              there is not any drift correction.  It is intended to be used in
              a startup script on systems with kernels above version 2.6 where
              you know the System Clock has been set from the  Hardware  Clock
              by the kernel during boot.

              It  does  the  following  things  that are detailed above in the
              --hctosys function:

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

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

              o Sets the kernel's timezone.

              The  first  two  are  only  available  on  the  first  call   of
              settimeofday(2) after boot.  Consequently this option only makes
              sense when used in a startup script.   If  the  Hardware  Clocks
              timescale  configuration  is  changed  then  a  reboot  would be
              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.

OPTIONS
       --adjfile=filename
              Override the default /etc/adjtime file path.

       --date=date_string
              This  option must be used with the --set or --predict functions,
              otherwise it is ignored.

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

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

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

       -D, --debug
              Display a lot of information about what hwclock is doing  inter-
              nally.   Some  of  its functions are complex and this output can
              help you understand how the program works.

       --directisa
              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.

       --epoch=year
              This option is required when using the --setepoch function.  The
              minimum year value is 1900.  The  maximum  is  system  dependent
              (ULONG_MAX - 1).

       -f, --rtc=filename
              Override  hwclock's  default rtc device file name.  Otherwise it
              will use the first one found in this order:
                  /dev/rtc0
                  /dev/rtc
                  /dev/misc/rtc
              For IA-64:
                  /dev/efirtc
                  /dev/misc/efirtc

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

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

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

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

       --noadjfile
              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-drift
              Update  the  Hardware  Clock's drift factor in /etc/adjtime.  It
              can only be used with --set or --systohc,

              A minimum four hour period between settings is  required.   This
              is  to  avoid  invalid calculations.  The longer the period, the
              more precise the resulting drift factor will be.

              This option was added in v2.26, because it is typical  for  sys-
              tems  to call hwclock --systohc at shutdown; with the old behav-
              iour this would automatically  (re)calculate  the  drift  factor
              which caused several problems:

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

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

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

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

              Having  hwclock  calculate  the  drift factor is a good starting
              point, but for  optimal  results  it  will  likely  need  to  be
              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  Hard-
              ware  Clock  is corrupted by a power failure.  In that case, the
              clock must first be set without this  option.   Despite  it  not
              working,  the resulting drift correction factor would be invalid
              anyway.

NOTES
   Clocks in a Linux System
       There are two types of date-time clocks:

       The Hardware Clock: This clock is an independent hardware device,  with
       its  own power domain (battery, capacitor, etc), that operates when the
       machine is powered off, or even unplugged.

       On an ISA compatible system, this clock is specified as part of the ISA
       standard.  A control program can read or set this clock only to a whole
       second, but it can also detect the edges of the 1 second  clock  ticks,
       so the clock actually has virtually infinite precision.

       This  clock is commonly called the hardware clock, the real time clock,
       the RTC, the BIOS clock, and the CMOS clock.  Hardware  Clock,  in  its
       capitalized form, was coined for use by hwclock.  The Linux kernel also
       refers to it as the persistent clock.

       Some non-ISA systems have a few real time clocks with only one of  them
       having  its  own  power  domain.   A very low power external I2C or SPI
       clock chip might be used with a backup battery as the hardware clock to
       initialize  a  more functional integrated real-time clock which is used
       for most other purposes.

       The System Clock: This clock is part of the Linux kernel and is  driven
       by  a timer interrupt.  (On an ISA machine, the timer interrupt is part
       of the ISA standard.)  It has meaning only while Linux  is  running  on
       the  machine.   The System Time is the number of seconds since 00:00:00
       January 1, 1970 UTC (or more succinctly, the number  of  seconds  since
       1969  UTC).   The System Time is not an integer, though.  It has virtu-
       ally infinite precision.

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

       It  is important that the System Time not have any discontinuities such
       as would happen if you used the date(1) program to  set  it  while  the
       system is running.  You can, however, do whatever you want to the Hard-
       ware Clock while the system is running, and the next time Linux  starts
       up,  it  will  do  so  with  the adjusted time from the Hardware Clock.
       Note:  currently  this  is  not  possible  on  most   systems   because
       hwclock --systohc is called at shutdown.

       The  Linux kernel's timezone is set by hwclock.  But don't be misled --
       almost nobody cares what timezone the kernel thinks it is in.  Instead,
       programs  that  care  about  the timezone (perhaps because they want to
       display a local time for you) almost  always  use  a  more  traditional
       method  of  determining the timezone: They use the TZ environment vari-
       able or the /etc/localtime file, as  explained  in  the  man  page  for
       tzset(3).   However, some programs and fringe parts of the Linux kernel
       such as filesystems use the kernel's timezone value.  An example is the
       vfat  filesystem.   If  the  kernel  timezone  value is wrong, the vfat
       filesystem will report and set the wrong timestamps on files.   Another
       example is the kernel's NTP '11 minute mode'.  If the kernel's timezone
       value and/or the persistent_clock_is_local variable are wrong, then the
       Hardware  Clock  will  be set incorrectly by '11 minute mode'.  See the
       discussion below, under Automatic Hardware Clock Synchronization by the
       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 local-
       ity at the present time.  This second field is not used under Linux and
       is always zero.  See also settimeofday(2).

   Hardware Clock Access Methods
       hwclock  uses many different ways to get and set Hardware Clock values.
       The most normal way is to do I/O to the rtc device special file,  which
       is presumed to be driven by the rtc device driver.  Also, Linux systems
       using the rtc framework with udev, are capable of  supporting  multiple
       Hardware Clocks.  This may bring about the need to override the default
       rtc device by specifying one with the --rtc option.

       However, this method is not always available as older  systems  do  not
       have  an  rtc  driver.   On  these systems, the method of accessing the
       Hardware Clock depends on the system hardware.

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

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

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

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

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

       When you use the --update-drift option with  --set  or  --systohc,  the
       systematic  drift  rate  is (re)calculated by comparing the fully drift
       corrected current Hardware Clock time with the new set time, from  that
       it  derives  the  24 hour drift rate based on the last calibrated time-
       stamp from the adjtime file.  This updated drift factor is  then  saved
       in /etc/adjtime.

       A  small  amount  of error creeps in when the Hardware Clock is set, so
       --adjust refrains from making any adjustment that is less than  1  sec-
       ond.   Later  on, when you request an adjustment again, the accumulated
       drift will be more than 1 second and --adjust will make the  adjustment
       including any fractional amount.

       hwclock --hctosys  also  uses  the  adjtime file data to compensate the
       value read from the Hardware Clock before using it to  set  the  System
       Clock.  It does not share the 1 second limitation of --adjust, and will
       correct sub-second drift values immediately.  It does  not  change  the
       Hardware  Clock time nor the adjtime file.  This may eliminate the need
       to use --adjust, unless something else on the system needs the Hardware
       Clock to be compensated.

   The Adjtime File
       While named for its historical purpose of controlling adjustments only,
       it actually contains other information used by hwclock from one invoca-
       tion to the next.

       The format of the adjtime file is, in ASCII:

       Line  1:  Three  numbers,  separated by blanks: 1) the systematic drift
       rate in seconds per day, floating point decimal; 2) the resulting  num-
       ber of seconds since 1969 UTC of most recent adjustment or calibration,
       decimal integer; 3) zero (for compatibility with clock(8)) as a decimal
       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  con-
       tain a valid time).  This is a decimal integer.

       Line  3:  "UTC" or "LOCAL".  Tells whether the Hardware Clock is set to
       Coordinated Universal Time or local time.  You can always override this
       value with options on the hwclock command line.

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

   Automatic Hardware Clock Synchronization by the Kernel
       You should be aware of another way that the Hardware Clock is kept syn-
       chronized  in  some  systems.   The  Linux kernel has a mode wherein it
       copies the System Time to the Hardware Clock  every  11  minutes.  This
       mode  is a compile time option, so not all kernels will have this capa-
       bility.  This is a good mode  to  use  when  you  are  using  something
       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 capa-
       bility  and the Hardware Clock timescale configuration is changed, then
       a reboot is required to notify the kernel.

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

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

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

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

DATE-TIME CONFIGURATION
   Keeping Time without External Synchronization
       This discussion is based on the following conditions:

       o Nothing  is running that alters the date-time clocks, such as ntpd(1)
         or a cron job.

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

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

       o During shutdown the following is called:
         hwclock --systohc

           * Systems without adjtimex may use ntptime.

       Whether  maintaining precision time with 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  fac-
       tors)  depends  upon  the System Clock's rate being correct, means that
       configuration of the System Clock should be done first.

       The System Clock drift is  corrected  with  the  adjtimex(8)  command's
       --tick  and  --frequency options.  These two work together: tick is the
       coarse adjustment and frequency is the fine adjustment.   (For  systems
       that  do  not  have  an  adjtimex  package,  ntptime -f ppm may be used
       instead.)

       Some Linux distributions attempt to automatically calculate the  System
       Clock  drift  with adjtimex's compare operation.  Trying to correct one
       drifting clock by using another drifting clock as a reference  is  akin
       to  a dog trying to catch its own tail.  Success may happen eventually,
       but great effort and frustration will likely precede it.  This  automa-
       tion  may  yield  an  improvement  over no configuration, but expecting
       optimum results would be in error.  A better choice for manual configu-
       ration would be adjtimex's --log options.

       It  may  be  more effective to simply track the System Clock drift with
       sntp, or date -Ins and a precision timepiece, and  then  calculate  the
       correction manually.

       After  setting  the  tick  and  frequency  values, continue to test and
       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  cor-
       rectly (step 6a) just before doing so.

       Having hwclock calculate the drift factor is a good starting point, but
       for optimal results it will likely need  to  be  adjusted  by  directly
       editing  the  /etc/adjtime file.  Continue to test and refine the drift
       factor until the Hardware Clock is corrected properly at  startup.   To
       check  this,  first  make  sure  that the System Time is correct before
       shutdown and then use sntp, or date -Ins  and  a  precision  timepiece,
       immediately after startup.

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

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

       o If  Linux  is  NOT  running during a daylight saving time change, the
         time read from the Hardware  Clock  will  NOT  be  adjusted  for  the
         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 oper-
       ate in the UTC timescale. The Hardware Clock's purpose is to initialize
       the System Clock, so also keeping it in UTC makes sense.

       Linux does, however, attempt to accommodate the Hardware Clock being in
       the local timescale. This is primarily for dual-booting with older ver-
       sions  of  MS  Windows. From Windows 7 on, the RealTimeIsUniversal reg-
       istry key is supposed to be working properly so that its Hardware Clock
       can be kept in UTC.

   POSIX vs 'RIGHT'
       A  discussion  on  date-time  configuration would be incomplete without
       addressing timezones, this is mostly well  covered  by  tzset(3).   One
       area  that  seems  to have no documentation is the 'right' directory of
       the Time Zone Database, sometimes called tz or zoneinfo.

       There are two separate databases in  the  zoneinfo  system,  posix  and
       'right'.  'Right'  (now named zoneinfo-leaps) includes leap seconds and
       posix does not. To use the 'right' database the System  Clock  must  be
       set  to  (UTC + leap  seconds), which is equivalent to (TAI - 10). This
       allows calculating the exact number of seconds between two  dates  that
       cross  a  leap  second epoch. The System Clock is then converted to the
       correct civil time, including UTC, by using the 'right' timezone  files
       which subtract the leap seconds. Note: this configuration is considered
       experimental and is known to have issues.

       To configure a system to use a particular database  all  of  the  files
       located   in   its   directory   must   be   copied   to  the  root  of
       /usr/share/zoneinfo.  Files are never used directly from the  posix  or
       'right' subdirectories, e.g., TZ='right/Europe/Dublin'.  This habit was
       becoming so common that the upstream zoneinfo project restructured  the
       system's  file  tree by moving the posix and 'right' subdirectories out
       of the zoneinfo directory and into sibling directories:

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

       Unfortunately, some Linux distributions are changing it back to the old
       tree  structure in their packages. So the problem of system administra-
       tors reaching into the 'right' subdirectory persists. This  causes  the
       system  timezone  to  be  configured  to include leap seconds while the
       zoneinfo database is still configured to exclude  them.  Then  when  an
       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.

EXIT STATUS
       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.

ENVIRONMENT
       TZ     If this variable is set its value takes precedence over the sys-
              tem configured timezone.

       TZDIR  If this variable is set its value takes precedence over the sys-
              tem configured timezone database directory path.

FILES
       /etc/adjtime
              The configuration and state file for hwclock.

       /etc/localtime
              The system timezone file.

       /usr/share/zoneinfo/
              The system timezone database directory.

       Device files hwclock may try for Hardware Clock access:
       /dev/rtc0
       /dev/rtc
       /dev/misc/rtc
       /dev/efirtc
       /dev/misc/efirtc

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

AUTHORS
       Written by Bryan Henderson, September  1996  (bryanh@giraffe-data.com),
       based  on  work  done  on  the clock(8) program by Charles Hedrick, Rob
       Hooft, and Harald Koenig.  See the source code for complete history and
       credits.

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

util-linux                         July 2017                        HWCLOCK(8)
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