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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 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/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_dst‐
              time 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 num‐
              ber  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 adj‐
              time file. This is useful for example if you need to set an RTC wakeup time to dis‐
              tant future and want to account for the RTC drift.

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

       -h, --help
              Display help text and exit.

       -V, --version
              Display version information 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 Coordi‐
              nated 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 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  success‐
              fully  run with the --set, --systohc, or --adjust options), as recorded in the adj‐
              time file.  If the adjtime 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 cal‐
              ibrated  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, 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).

       --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 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.)

              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 Hard‐
              ware 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  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‐
       sion.

       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 inte‐
       grated 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 succinctly, the number of seconds
       since 1969).  The System Time is not an integer, though.  It has virtually infinite preci‐
       sion.

       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  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/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 time‐
       stamps 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_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).)


Users access and setuid
       Sometimes, you need to install hwclock setuid root. If you want users other than the supe‐
       ruser 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 hang‐
       ing).  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  hard‐
       ware.

       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 (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  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
       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 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
       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 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.

       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.


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 AT 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.ker‐
       nel.org/pub/linux/utils/util-linux/.



util-linux                                 August 2011                                 HWCLOCK(8)


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