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HWCLOCK(8)		     System Administration		    HWCLOCK(8)

       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 con-
       sistently 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/zone-
	      info, 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

       -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 ker-
	      nel'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. If the Hardware Clock is already in UTC, it
	      is not reset.

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

	      Print  the  kernel's Hardware Clock epoch value to standard out-
	      put.  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 speci-
	      fied by the --epoch  option.   See  the  --getepoch  option  for

	      Predict  what  the  RTC  will  read  at time given by the --date
	      option based on the adjtime file. This is useful for example  if
	      you need to set an RTC wakeup time to distant future and want to
	      account for the RTC drift.

       -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.

       -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  oth-
       ers apply to most functions.

	      You need this option if you specify the --set or --predict func-
	      tions, 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 Hard-
	      ware 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 Univer-
	      sal 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 nei-
	      ther and take a wrong default), both setting and querying of the
	      Hardware Clock will be messed up.

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

	      Disables the facilities provided by /etc/adjtime.	 hwclock  will
	      not  read nor write to that file with this option.  Either --utc
	      or --localtime must be specified when using this option.

	      Overrides the default /etc/adjtime.

       -f, --rtc=filename
	      Overrides the default /dev file name, which is /dev/rtc on  many
	      platforms but may be /dev/rtc0, /dev/rtc1, and so on.

	      This  option  is	meaningful  only on an ISA machine or an Alpha
	      (which implements enough of ISA to be, roughly speaking, an  ISA
	      machine  for hwclock's purposes).	 For other machines, it has no
	      effect.  This option tells hwclock to use explicit I/O  instruc-
	      tions  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  instruc-
	      tions 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 can-
	      not 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  date  is  within the past year.  For this to work, you had
	      better do a hwclock --set or hwclock --systohc at least  once  a

	      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 set-
	      ting it, this scheme could be defeated and you could end up los-
	      ing 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	deter-
	      mine  by	itself	what  it's  running on, at least when /proc is
	      mounted.	(If you find you need one of  these  options  to  make
	      hwclock  work,  contact the maintainer to see if the program can
	      be improved to detect  your  system  automatically.   Output  of
	      `hwclock --debug' and `cat /proc/cpuinfo' may be of interest.)

	      Option  --jensen	means  you are running on a Jensen model.  And
	      --funky-toy means that on your machine one has to use the UF bit
	      instead  of  the	UIP bit in the Hardware Clock to detect a time
	      transition.  "Toy" in the option name refers to the Time Of Year
	      facility of the machine.

       --test Do  everything  except  actually	updating the Hardware Clock or
	      anything else.  This is useful, especially in  conjunction  with
	      --debug, in learning about hwclock.

	      Display  a lot of information about what hwclock is doing inter-
	      nally.  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 con-
       trol 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	exter-
       nal  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 ker-
       nel 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 sec-
       onds 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 Hard-
       ware  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	 (per-
       haps  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_min-
       uteswest	 indicating how many minutes local time (not adjusted for DST)
       lags behind UTC, and 2) a field tz_dsttime indicating the type of  Day-
       light  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).)

Users access and setuid
       Sometimes,  you	need to install hwclock setuid root. If you want users
       other than the superuser to be able to display the  clock  value	 using
       the  direct  ISA	 I/O  method,  install it setuid root. If you have the
       /dev/rtc interface on your system or are on a non-ISA  system,  there's
       probably	 no  need for users to use the direct ISA I/O method, so don't

       In any case, hwclock will not allow you to set anything unless you have
       the  superuser  real  uid. (This is restriction is not necessary if you
       haven't installed setuid root, but it's there for now).

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 speci-
       fying /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" regis-
       ters 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 instruc-
       tions, 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 rea-
       sons 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 --direc-
       tisa 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 com-
       mand to set the Hardware Clock to the true current time.	 Hwclock  cre-
       ates  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 run-
       ning 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 inte-
       ger; 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 syn-
       chronized 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 Sys-
       tem 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 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/  (/usr/lib/zoneinfo  on old systems)
       /dev/rtc /dev/rtc0 /dev/port /dev/tty1 /proc/cpuinfo

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

       Written by Bryan Henderson, September  1996  (bryanh@giraffe-data.com),
       based  on work done on the clock program by Charles Hedrick, Rob Hooft,
       and Harald Koenig.  See the source code for complete history and	 cred-

       The  hwclock command is part of the util-linux package and is available
       from ftp://ftp.kernel.org/pub/linux/utils/util-linux/.

util-linux			  August 2011			    HWCLOCK(8)