Man Linux: Main Page and Category List


       hwclock - query and set the hardware clock (RTC)


       hwclock [functions] [options]


       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
       consistently 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:

       -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

       --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 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
              specified  by the --epoch option.  See the --getepoch option for

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

              You  need  this  option  if  you  specify  the   --set   option.
              Otherwise,  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.

       -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 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 /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.

              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
              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
              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.)

              --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
              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-ng  package and is
       available from

                                06 August 2008