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 consistently 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/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.

              Reset the System Time based on the current timezone.

              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  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.

              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  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.

              Set  the  kernel's  Hardware  Clock  epoch  value  to  the value specified 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.

       -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 others apply to most functions.

              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.

              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 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 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 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.

              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
              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.

              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
              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
              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 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 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 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

       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 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 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.  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 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).)

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 --directisa 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: 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  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.

       To  see  if  it  is  on  or  off,  use  the command adjtimex --print and look at the value of "status".  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.   ACPI  provides  a
       standard way to access century values, when they are supported by the hardware.




       /etc/adjtime /usr/share/zoneinfo/ /dev/rtc /dev/rtc0 /dev/port /dev/tty1 /proc/cpuinfo


       adjtimex(8),   date(1),   gettimeofday(2),   settimeofday(2),   crontab(1),   tzset(3)   /etc/init.d/hwclock.sh,   /usr/share/doc/util-


       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.


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

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