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    Command:

    hwclock

    
    
    

    SYNOPSIS

           hwclock [functions] [options]
    
    
    

    DESCRIPTION

           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 con-
           sistently gains or loses time at a certain rate if left to run).
    
    
    

    FUNCTIONS

           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.
    
           --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
                  scripts.
    
           -w, --systohc
                  Set the Hardware Clock to the current System Time.
    
           --systz
                  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)
    
           --getepoch
                  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 Counter epoch value must be 1952.
    
                  This  epoch  value  is  used  whenever hwclock reads or sets the
                  Hardware Clock.
    
           --setepoch
                  Set the kernel's Hardware Clock epoch value to the value  speci-
                  fied  by  the  --epoch  option.   See  the --getepoch option for
                  details.
    
           -v, --version
                  Print the version of hwclock on Standard Output.
    
           --date=date_string
                  You need this option if you specify the  --set  option.   Other-
                  wise,  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.
    
           --epoch=year
                  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
    
    
    

    OPTIONS

           The following options apply to most functions.
    
           -u, --utc
    
           --localtime
                  file.   If  the adjtime file doesn't exist, the default is local
                  time.
    
           --noadjfile
                  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.
    
           --adjfile=filename
                  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.
    
           --directisa
                  is  meaningful  only on an ISA machine or an Alpha (which imple-
                  ments 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.
    
           --badyear
                  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 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!
    
                  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).
    
           --jensen
    
           --funky-toy
                  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.)
    
                  --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.
    
           --debug
                  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.
    
    
    

    NOTES

    
    

    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
           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,
           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
           option.
    
           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).)
    
    
    

    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-
           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.
    
    
           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.
    
    
    

    SEE ALSO

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

    AUTHORS

           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-
           its.
    
    
    

    AVAILABILITY

           The  hwclock command is part of the util-linux-ng package and is avail-
           able from ftp://ftp.kernel.org/pub/linux/utils/util-linux-ng/.
    
                                    06 August 2008                      HWCLOCK(8)
    
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