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

              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

              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 details.

              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

              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 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).


              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

       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 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

       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

       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

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

       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

       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

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

       Written by Bryan Henderson, September 1996 (,
       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

util-linux                         August 2011                        HWCLOCK(8)