Provided by: util-linux_2.12r-4ubuntu6_i386 bug


       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
       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:

       --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/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

              Set the Hardware Clock to the current System 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

              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.


              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.

              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.

       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).

       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, 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.




       /etc/adjtime   /usr/share/zoneinfo/   /dev/rtc   /dev/port    /dev/tty1


       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

                                 02 March 1998                      HWCLOCK(8)