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RedHat 9 (Linux i386) - man page for hwclock (redhat section 8)

HWCLOCK(8)			     System Manager's Manual			       HWCLOCK(8)

       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 option.

       --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 settimeofday(2).)

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

	      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  out  standard  output  the kernel's Hardware Clock epoch value.  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 details.

	      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 speci-
	      fied 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.

	      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

	      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 cal-
	      ibrated 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, which-
	      ever 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 automat-
	      ically. 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 off.

       On an ISA system, this clock is specified as part of the ISA standard.  The  control  pro-
       gram 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 preci-

       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 preci-

       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  mis-
       led  --	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/local/timezone 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 time-
       stamps on files.

       hwclock sets the kernel timezone to the value indicated by TZ  and/or  /usr/local/timezone
       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_dst-
       time  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  hang-

       On  older  systems, the method of accessing the Hardware Clock depends on the system hard-

       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 pro-
       vides 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 manipula-
       tion of the CMOS registers without even trying  /dev/rtc  by  specifying  the  --directisa

The Adjust Function
       The  Hardware Clock is usually not very accurate.  However, much of its inaccuracy is com-
       pletely 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  infor-
       mation.	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  sec-
       onds.   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  Univer-
       sal  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 sophisti-
       cated 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 "sta-
       tus".   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/ (/usr/lib/zoneinfo on old  systems)  /dev/rtc	/dev/port
       /dev/tty1 /proc/cpuinfo

       adjtimex(8), 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 credits.

					  02 March 1998 			       HWCLOCK(8)

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