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HWCLOCK(8)                                                          HWCLOCK(8)

       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

       You  can  also run hwclock periodically to insert or remove time from the Hardware Clock to compensate for sys-
       tematic 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 option.

       -c, --compare
              Periodically  compare  the Hardware Clock to the System Time and output the difference every 10 seconds.
              This will also print the frequency offset and tick.

       --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 ker-
              nel's timezone value is set to DST_NONE. (For details on what this field used to  mean,  see  settimeof-

              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.

              Set the kernel's timezone and reset the System Time based on the current timezone.

              The system time is only reset on the first call after boot.

              The  local  timezone  is  taken  to  be  what  is  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 sys-
              tem startup scripts for recent 2.6 kernels where you know the System Time contains  the  Hardware  Clock
              time. If the Hardware Clock is already in UTC, it is not reset.

              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 ker-
              nel'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 details.

       -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) pro-
              gram.  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  set-
              ting and querying of the Hardware Clock will be messed up.

              If  you  specify  neither  --utc  nor --localtime , the default is whichever was specified the last time
              hwclock was used to set the clock (i.e. hwclock was successfully  run  with  the  --set,  --systohc,  or
              --adjust  options),  as recorded in the adjtime file.  If the adjtime file doesn't exist, the default is
              local time.

              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  speak-
              ing,  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

       --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 auto-
              matically. 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

       --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 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 suc-
       cinctly, the number of seconds since 1969).  The System Time is not an integer, though.  It has virtually infi-
       nite 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.

       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 Sys-
       tem 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 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  run-
       ning  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 avail-
       able 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  com-
       mand 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 period-
       ically 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 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.

       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 cen-
       tury 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 appropri-

       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/ (/usr/lib/zoneinfo on old systems)  /dev/rtc  /dev/rtc0  /dev/port  /dev/tty1

       date(1), gettimeofday(2), settimeofday(2), crontab(1), tzset(3)

       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-ng  package   and   is   available   from   ftp://ftp.ker-

                                06 August 2008                      HWCLOCK(8)