HWCLOCK(8) HWCLOCK(8)

NAME hwclock - query and set the hardware clock (RTC)

SYNOPSIS 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.

DESCRIPTION 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).

OPTIONS 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
          option.


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

   --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
          scripts.

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

   --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 discussion below.

   --getepoch
          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.

   --setepoch
          Set  the kernel’s Hardware Clock epoch value to the value speci?
          fied by the --epoch  option.   See  the  --getepoch  option  for
          details.

   --version
          Print the version of hwclock on Standard Output.

   --date=date_string
          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.


   --epoch=year
          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.

   --utc

   --localtime
          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
          time.


   --noadjfile
          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.


   --directisa
          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.

   --badyear
          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).

   --jensen

   --funky-toy
          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.

   --debug
          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.

NOTES 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
   off.

   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.

   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
   option.

   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).

   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 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, 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.

ENVIRONMENT VARIABLES TZ

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

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

AUTHORS 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? its.

                             02 March 1998                      HWCLOCK(8)

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