hwclock [function] [option...]

       hwclock  is  a  tool for accessing the Hardware Clock.  You can display
       the current time, set the Hardware Clock to a specified time,  set  the
       Hardware  Clock  from  the System Time, or set the System Time from the
       Hardware Clock.

       You can also run hwclock periodically to add or subtract time from  the
       Hardware Clock to compensate for systematic drift (where the clock con-
       sistently loses or gains time at a certain rate when left to run).

       You need exactly one of the following  options  to  tell  hwclock  what
       function to perform:

       -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  Coordinated  Universal Time.  See the --utc
              option.  Showing the Hardware Clock time is the default when  no
              function is specified.

       --set  Set the Hardware Clock to the time given by the --date option.

       -s, --hctosys
              Set the System Time from the Hardware Clock.

              Also  set  the  kernel's timezone value to the local timezone as
              indicated by the TZ environment variable and/or /usr/share/zone-
              info, as tzset(3) would interpret them.  The obsolete tz_dsttime
              field of the kernel's timezone value is set to  DST_NONE.   (For
              details on what this field used to mean, see settimeofday(2).)

              This  is  a  good  option  to  use  in one of the system startup

       -w, --systohc
              Set the Hardware Clock to the current System Time.

              Reset the System Time based on the current timezone.

              Also set the kernel's timezone value to the  local  timezone  as
              indicated by the TZ environment variable and/or /usr/share/zone-
              info, as tzset(3) would interpret them.  The obsolete tz_dsttime
              field  of  the kernel's timezone value is set to DST_NONE.  (For
              details on what this field used to mean, see settimeofday(2).)

              put.  This 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.

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

              Set  the kernel's Hardware Clock epoch value to the value speci-
              fied by the --epoch  option.   See  the  --getepoch  option  for

              Predict  what  the  RTC  will  read  at time given by the --date
              option based on the adjtime file. This is useful for example  if
              you need to set an RTC wakeup time to distant future and want to
              account for the RTC drift.

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

       -V, --version
              Display the version of hwclock and exit.

       The first two options apply to just a few specific functions, the  oth-
       ers apply to most functions.

              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 an argument
              to the date(1) program.  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 Coordinated Universal time.  See the --utc option.

              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, or otherwise to specify
              the epoch for use with direct ISA access.

              For example, on a Digital Unix machine:

              If  you  specify the wrong one of these options (or specify nei-
              ther and take a wrong default), both setting and querying of the
              Hardware Clock will be messed up.

              If  you  specify  neither  --utc nor --localtime, the default is
              whichever was specified the last time hwclock was  used  to  set
              the  clock  (i.e.   hwclock was successfully run with the --set,
              --systohc, or --adjust options),  as  recorded  in  the  adjtime
              file.   If  the  adjtime  file doesn't exist, the default is UTC

              Disables 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.

              Overrides the default /etc/adjtime.

       -f, --rtc=filename
              Overrides the default /dev file name, which is /dev/rtc on  many
              platforms but may be /dev/rtc0, /dev/rtc1, and so on.

              This  option  is  meaningful  only on an ISA machine or an Alpha
              (which implements enough of ISA to be, roughly speaking, an  ISA
              machine  for hwclock's purposes).  For other machines, it has no
              effect.  This option tells hwclock to use explicit I/O  instruc-
              tions  to  access  the  Hardware  Clock.   Without  this option,
              hwclock will try to use the /dev/rtc device (which it assumes to
              be  driven  by  the RTC device driver).  If it is unable to open
              the device (for reading), it will use the explicit I/O  instruc-
              tions anyway.

              Indicates  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
              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.

              hwclock warns you that you probably need --badyear  whenever  it
              finds your Hardware Clock set to 1994 or 1995.

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

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


              These two options specify what kind of Alpha machine  you  have.
              They  are  invalid  if  you  don't have an Alpha and are usually
              unnecessary if you do, because hwclock should be able to  deter-
              mine  by  itself  what  it's  running on, at least when /proc is
              mounted.  (If you find you need one of  these  options  to  make
              hwclock  work,  contact the maintainer to see if the program can
              be improved to detect  your  system  automatically.   Output  of
              `hwclock --debug' and `cat /proc/cpuinfo' may be of interest.)

              Option  --jensen  means  you are running on a Jensen model.  And
              --funky-toy means that on your machine one has to use the UF bit
              instead  of  the  UIP bit in the Hardware Clock to detect a time
              transition.  "Toy" in the option name refers to the Time Of Year
              facility of the machine.

       --test Do  everything  except  actually  updating the Hardware Clock or
              anything else.  This is useful, especially in  conjunction  with
              --debug, in learning about hwclock.

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

Clocks in a Linux System
       There are two main clocks in a Linux system:

       The Hardware Clock: This is a clock that runs independently of any con-
       trol program running in the CPU and even when the  machine  is  powered

       nal  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 Time: This is the time kept by a clock inside the Linux ker-
       nel and 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 sec-
       onds since 00:00:00 January 1, 1970 UTC (or more succinctly, the number
       of seconds since 1969).  The System Time is not an integer, though.  It
       has virtually infinite precision.

       The  System  Time is the time that matters.  The Hardware Clock's basic
       purpose in a Linux system is to keep time when Linux  is  not  running.
       You initialize the System Time to the time from the Hardware Clock when
       Linux starts up, and then never use the  Hardware  Clock  again.   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(1L) 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.   You
       can also use the program adjtimex(8) to smoothly adjust the System Time
       while the system runs.

       A Linux kernel maintains a concept of a local timezone for the  system.
       But  don't  be  misled  -- almost nobody cares what timezone the kernel
       thinks it is in.  Instead, programs that care about the timezone  (per-
       haps  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  and/or the /usr/share/zoneinfo directory, 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
       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.

       hwclock sets the kernel timezone to the value indicated  by  TZ  and/or
       /usr/share/zoneinfo  when  you  set the System Time using the --hctosys

       The timezone value actually consists of two parts: 1) a  field  tz_min-
       uteswest  indicating how many minutes local time (not adjusted for DST)
       lags behind UTC, and 2) a field tz_dsttime indicating the type of  Day-
       light  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).)

How hwclock Accesses the Hardware Clock
       hwclock  uses many different ways to get and set Hardware Clock values.
       The most normal way is to do I/O to the device special  file  /dev/rtc,

       On an ISA system, hwclock can directly access the "CMOS memory"  regis-
       ters  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.  (In the case of a
       Jensen Alpha, there is no way for hwclock to execute those I/O instruc-
       tions,  and so it uses instead the /dev/port device special file, which
       provides almost as low-level an interface to the I/O subsystem).

       This is a really poor method of accessing the clock, for all  the  rea-
       sons  that  user space programs are generally not supposed to do direct
       I/O and disable interrupts.  Hwclock provides it because it is the only
       method  available on ISA and Alpha systems which don't have working rtc
       device drivers available.

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

       hwclock  tries  to  use  /dev/rtc.  If it is compiled for a kernel that
       doesn't have that function or it is unable to  open  /dev/rtc  (or  the
       alternative  special  file  you've defined on the command line) hwclock
       will fall back to another method, if available.  On  an  ISA  or  Alpha
       machine,  you  can  force hwclock to use the direct manipulation of the
       CMOS registers without even trying /dev/rtc by specifying the  --direc-
       tisa option.

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 make systematic corrections to  correct  the
       systematic drift.

       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.  5 days later, the clock has gained 10
       seconds, so you issue another hwclock --set 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 goes 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

       It is good to do a hwclock --adjust just before the  hwclock  --hctosys
       at system startup time, and maybe periodically while the system is run-
       ning via cron.

       The adjtime file, while named for its historical purpose of controlling
       adjustments  only,  actually  contains  other  information  for  use by
       hwclock in remembering information from one invocation to the next.

       The format of the adjtime file is, in ASCII:

       Line 1: 3 numbers, separated by blanks: 1)  systematic  drift  rate  in
       seconds per day, floating point decimal; 2) Resulting number of seconds
       since 1969 UTC of most recent adjustment or calibration, decimal  inte-
       ger; 3) zero (for compatibility with clock(8)) as a decimal integer.

       Line  2:  1  number: 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 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 is
       a  good mode to use when you are using something sophisticated like ntp
       to keep your System Time synchronized. (ntp is a way to keep your  Sys-
       tem  Time synchronized either to a time server somewhere on the network
       or to a radio clock hooked up to your system.  See RFC 1305).

       This mode (we'll call it "11 minute mode") is off until something turns
       it  on.   The  ntp daemon xntpd is one thing that turns it on.  You can
       turn it off by running anything, including hwclock --hctosys, that sets
       the System Time the old fashioned way.

       To see if it is on or off, use the command adjtimex --print and look at
       the value of "status".  If the "64" bit of this  number  (expressed  in
       binary) equal to 0, 11 minute mode is on.  Otherwise, it is off.

       If  your system runs with 11 minute mode on, don't use hwclock --adjust
       or hwclock --hctosys.  You'll just make a mess.  It  is  acceptable  to
       use a hwclock --hctosys at startup time to get a reasonable System Time
       until your system is able to set the  System  Time  from  the  external
       source and start 11 minute mode.

       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.


       /etc/adjtime   /usr/share/zoneinfo/   /dev/rtc   /dev/rtc0    /dev/port
       /dev/tty1 /proc/cpuinfo

       adjtimex(8),  date(1),  gettimeofday(2),  settimeofday(2),  crontab(1),
       tzset(3)          /etc/init.d/hwclock.sh,          /usr/share/doc/util-

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

       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                        August 2011                       HWCLOCK(8)
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