HWCLOCK(8)                  System Administration                 HWCLOCK(8)

NAME         top

       hwclock - query or set the hardware clock (RTC)

SYNOPSIS         top

       hwclock [function] [option...]

DESCRIPTION         top

       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

FUNCTIONS         top

       You need exactly one of the following options to tell hwclock what
       function to perform:

              Add or subtract time from the Hardware Clock to account for
              systematic drift since the last time the clock was set or
              adjusted.  See the discussion below, under The Adjust

       -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.

              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

              Predict what the RTC will read at the 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 a distant future and
              want to account for the RTC drift.

       -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.

       -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.

       --set  Set the Hardware Clock to the time given by the --date option.

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

              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

       -w, --systohc
              Set the Hardware Clock to the current System Time.

       -V, --version
              Display version information and exit.

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

OPTIONS         top

              Override the default /etc/adjtime.

       --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).

              Indicate 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.

              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.

              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.

              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.

              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

       -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.


              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

       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.

              Indicate that the Hardware Clock is kept in local time.

       It is your choice whether to keep your clock in UTC or in local time,
       but nothing in the clock itself says which alternative you've chosen.
       So with --localtime or --utc you give this information to hwclock.
       If you specify the wrong one (or specify neither and take a wrong
       default), both setting and querying the Hardware Clock will be messed

       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.

              Disable 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.

       --srm  This option is equivalent to --epoch=1900 and is used to
              specify the most common epoch on Alphas with SRM console.

       --test Do everything except actually updating the Hardware Clock or
              anything else.  This is useful, especially in conjunction with
              --debug, in learning about

       -u, --utc
              Indicate that the Hardware Clock is kept in Coordinated
              Universal Time.  See the discussion under --localtime.

NOTES         top

   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

       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).)

   User 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 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

       This is a really poor method of accessing the clock, for all the
       reasons that userspace 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

       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.

       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



FILES         top

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

SEE ALSO         top

       date(1), gettimeofday(2), settimeofday(2), crontab(1), tzset(3)

AUTHORS         top

       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.

AVAILABILITY         top

       The hwclock command is part of the util-linux package and is
       available from

COLOPHON         top

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util-linux                        July 2014                       HWCLOCK(8)