Provided by: util-linux_2.20.1-1ubuntu3_i386 bug


       hwclock - query or set the hardware clock (RTC)


       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
       consistently 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/zoneinfo,  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

              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/zoneinfo,  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

              This is an alternate option to --hctosys that does not read  the
              hardware  clock,  and  may be used in system startup scripts for
              recent 2.6 kernels where you know the System Time  contains  the
              Hardware Clock time.

              Add  or  subtract  time  from  the Hardware Clock to account for
              systematic drift since the  last  time  the  clock  was  set  or
              adjusted.  See discussion below.

              Print  the  kernel's  Hardware  Clock  epoch  value  to standard
              output.  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
              specified 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
       others apply to most functions.

              You  need  this  option  if  you  specify the --set or --predict
              functions, 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
              Hardware Clock in Coordinated Universal  time.   See  the  --utc

              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:

                  hwclock --setepoch --epoch=1952

       -u, --utc

              Indicates  that  the  Hardware  Clock  is  kept  in  Coordinated
              Universal  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
              neither  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
              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  reading),  it  will  use  the  explicit  I/O
              instructions 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
              cannot  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
              setting it, this scheme could be defeated and you could  end  up
              losing 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
              determine 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
              internally.  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
       control 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
       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 Time: This is the time kept by  a  clock  inside  the  Linux
       kernel  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
       seconds 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
       Hardware 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
       (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  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_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
       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
       specifying /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"
       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.  (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
       reasons 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
       --directisa 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
       command to set the Hardware Clock to the true  current  time.   Hwclock
       creates 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
       running 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
       integer; 3)  zero  (for  compatibility  with  clock(8))  as  a  decimal

       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
       synchronized  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
       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).

       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


       The hwclock command is part of the util-linux package and is  available