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HWCLOCK(8)                                                          HWCLOCK(8)

       hwclock - query and set the hardware clock (RTC)

       hwclock -r or hwclock --show
       hwclock -w or hwclock --systohc
       hwclock -s or hwclock --hctosys
       hwclock -a or hwclock --adjust
       hwclock -v or hwclock --version
       hwclock --set --date=newdate
       hwclock --getepoch
       hwclock --setepoch --epoch=year

       other options:

       [-u|--utc]   --localtime   --noadjfile  --directisa --test [-D|--debug]

       and arcane options for DEC Alpha:

       [-A|--arc] [-J|--jensen] [-S|--srm] [-F|--funky-toy]

       Minimum unique abbreviations of all options are acceptable.

       Also, -h asks for a help message.

       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 to the System Time, and set the  System  Time  from  the
       Hardware Clock.

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

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

       --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

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

              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

              Set the Hardware Clock to the current System Time.

              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 discussion below.

              Print the kernel's Hardware Clock epoch value to  standard  out-
              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 Counter 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

              Print the version of hwclock on Standard Output.

              You need this option if you specify the  --set  option.   Other-
              wise,  it  is  ignored.  This specifies the time to which to set
              the Hardware Clock.  The value of this option is an argument  to
              the date(1) program.  For example,

              hwclock --set --date="9/22/96 16:45:05"

              The  argument  is  in local time, even if you keep your Hardware
              Clock in Coordinated Universal time.  See the --utc option.

              Specifies the year  which  is  the  beginning  of  the  Hardware
              Clock's epoch.  I.e. 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:

              hwclock --setepoch --epoch=1952

       The following options apply to most functions.


              Indicates that the Hardware Clock is kept in Coordinated Univer-
              sal Time or local time, respectively.  It is your choice whether
              to  keep  your  clock  in  UTC or local time, but nothing in the
              clock tells which you've chosen.  So this option is how you give
              that information to hwclock.

              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 local

              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 /dev file name, which is /dev/rtc on  many
              platforms but may be /dev/rtc0, /dev/rtc1, and so on.

              is  meaningful  only on an ISA machine or an Alpha (which imple-
              ments 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  instructions  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
              read), it will use the explicit I/O instructions anyway.

              The rtc device driver was new in Linux Release 2.

              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
              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 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.

              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.)

              --jensen means you are running on a Jensen model.

              --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

       On  an ISA system, this clock is specified as part of the ISA standard.
       The control program can read or set this clock to a whole  second,  but
       the  control  program  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  because  all  of  the
       other names are inappropriate to the point of being misleading.

       So  for  example, some non-ISA systems have a few real time clocks with
       only one of them having its own power domain.  A very low power  exter-
       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,
       which is presumed to be driven by the rtc device driver.  However, this
       method is not always available.  For one thing, the  rtc  driver  is  a
       relatively  recent  addition  to  Linux.   Older systems don't have it.
       Also, though there are versions of the rtc  driver  that  work  on  DEC
       Alphas,  there  appear  to  be plenty of Alphas on which the rtc driver
       does not work (a common symptom is hwclock hanging).  Moreover,  recent
       Linux  systems  have  more  generic support for RTCs, even systems that
       have more than one, so you might need to override the default by speci-
       fying /dev/rtc0 or /dev/rtc1 instead.

       On older systems, the method of accessing the Hardware Clock depends on
       the system hardware.

       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
       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).

       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
       clock was adjusted.

       Every time you calibrate (set) the clock (using --set or  --systohc  ),
       hwclock recalculates the systematic drift rate based on how long it has
       been since the last calibration, how long it has been  since  the  last
       adjustment, what drift rate was assumed in any intervening adjustments,
       and the amount by which the clock is presently off.

       A small amount of error creeps in any time hwclock sets the  clock,  so
       it refrains from making an adjustment that would be less than 1 second.
       Later on, when you request an adjustment again, the  accumulated  drift
       will be more than a second and hwclock will do the adjustment then.

       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.

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.


       /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/,          /usr/share/doc/util-

       Written  by  Bryan Henderson, September 1996 (,
       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-ng package and is  avail-
       able from

                                 02 March 1998                      HWCLOCK(8)

Time taken: 0.00048 seconds

Created with the man page lookup class by Andrew Collington,