HWCLOCK(8)						       System Administration							HWCLOCK(8)

NAME

       hwclock - query or set the hardware clock (RTC)

SYNOPSIS

       hwclock [function] [option...]

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 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).

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 Hard-
	      ware 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.

       --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) 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. If the Hardware Clock is already  in  UTC,  it  is  not
	      reset.

       --adjust
	      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.

       --getepoch
	      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.

       --setepoch
	      Set the kernel's Hardware Clock epoch value to the value specified by the --epoch option.  See the --getepoch option for details.

       --predict
	      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.

OPTIONS

       The first two options apply to just a few specific functions, the others apply to most functions.

       --date=date_string
	      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.

       --epoch=year
	      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

       --localtime
	      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  Hard-
	      ware 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  adj-
	      time file doesn't exist, the default is UTC 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
	      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.

       --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 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 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).

       --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 unneces-
	      sary 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 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.

       --debug
	      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.

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 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 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 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 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).)

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 bother.

       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
       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 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 --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 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: sub-
       tracts 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 adjust-
       ments, 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  sec-
       ond.   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 con-
       tain 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 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  stan-
       dard way to access century values, when they are supported by the hardware.

ENVIRONMENT VARIABLES

       TZ

FILES

       /etc/adjtime /usr/share/zoneinfo/ (/usr/lib/zoneinfo on old systems) /dev/rtc /dev/rtc0 /dev/port /dev/tty1 /proc/cpuinfo

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 credits.

AVAILABILITY

       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)