Provided by: util-linux_2.20.1-5.1ubuntu20_amd64 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 settimeofday(2).)

              This is a good option to use in one of the system startup scripts.

       -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 settimeofday(2).)

              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 details.

              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

              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 option.

              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 time.

              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

              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 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

       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

       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 option.

       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

       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

       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 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

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-linux/README.Debian.hwclock


       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 credits.


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