hwclock

HWCLOCK(8)                   System Administration                  HWCLOCK(8)

NAME
       hwclock - read or set the hardware clock (RTC)

SYNOPSIS
       hwclock [function] [option...]

DESCRIPTION
       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 below.

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

       --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.

       -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.

       --getepoch
       --setepoch
              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  con-
              vention  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
              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 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 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 (re)calculate the drift factor.

       --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.  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.

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

       --badyear
              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 can-
              not 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 set-
              ting it, this scheme could be defeated and you could end up los-
              ing a day.

       --date=date_string
              You need this option if you specify the --set or --predict func-
              tions,  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  Hard-
              ware  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  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 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 condi-
              tion 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:
                  /dev/rtc
                  /dev/rtc0
                  /dev/misc/rtc
              For IA-64:
                  /dev/efirtc
                  /dev/misc/efirtc

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

       --update-drift
              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 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.

              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.

OPTIONS FOR ALPHA MACHINES ONLY
       --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).

       --epoch=year
              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

       --funky-toy
       --jensen
              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.

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
       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 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.

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/rtc
       /dev/rtc0
       /dev/misc/rtc
       /dev/efirtc
       /dev/misc/efirtc
       /dev/port
       /dev/tty1

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 ftp://ftp.kernel.org/pub/linux/utils/util-linux/.

util-linux                        April 2015                        HWCLOCK(8)
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