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.TH HWCLOCK 8 "06 August 2008"
hwclock \- query and set the hardware clock (RTC)
.B hwclock
.RI [ functions ]
.RI [ options ]
.B 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 to the System Time, and set the System Time from the
Hardware Clock.
You can also run
.B hwclock
periodically to insert or remove time from the Hardware Clock to
compensate for systematic drift (where the clock consistently gains or
loses time at a certain rate if left to run).
You need exactly one of the following options to tell
.B hwclock
what function to perform:
.BR \-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
.B \-\-utc
.B \-\-set
Set the Hardware Clock to the time given by the
.B \-\-date
.BR \-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
.IR /usr/share/zoneinfo ,
.BR 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
.BR settimeofday (2).)
This is a good option to use in one of the system startup scripts.
.BR \-w , \ \-\-systohc
Set the Hardware Clock to the current System Time.
.B \-\-systz
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
.IR /usr/share/zoneinfo ,
.BR 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
.BR settimeofday (2).)
This is an alternate option to
.B \-\-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.
.B \-\-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
.B \-\-getepoch
Print the kernel's Hardware Clock epoch value to standard output.
This is the number of years into AD to which a zero year value in the
Hardware Clock refers. For example, if you are using the convention
that the year counter in your Hardware Clock contains the number of
full years since 1952, then the kernel's Hardware Counter epoch value
must be 1952.
This epoch value is used whenever hwclock reads or sets the Hardware Clock.
.B \-\-setepoch
Set the kernel's Hardware Clock epoch value to the value specified by the
.B \-\-epoch
option. See the
.B \-\-getepoch
option for details.
.BR \-v , \ \-\-version
Print the version of
.B hwclock
on Standard Output.
.BI \-\-date= date_string
You need this option if you specify the
.B \-\-set
option. Otherwise, it is ignored.
This specifies the time to which to set the Hardware Clock.
The value of this option is an argument to the
.BR date (1)
For example,
.I hwclock --set --date="9/22/96 16:45:05"
The argument is in local time, even if you keep your Hardware Clock in
Coordinated Universal time. See the
.B \-\-utc
.BI \-\-epoch= year
Specifies the year which is the beginning of the Hardware Clock's
epoch. I.e. 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:
.I hwclock --setepoch --epoch=1952
The following options apply to most functions.
.BR \-u , \ \-\-utc
.B \-\-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
.BR 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
.B \-\-utc
.B \-\-localtime
, the default is whichever was specified the last time
.B hwclock
was used to set the clock (i.e. hwclock was successfully run with the
.BR \-\-set ,
.BR \-\-systohc ,
.B \-\-adjust
options), as recorded in the adjtime file. If the adjtime file doesn't
exist, the default is local time.
.B \-\-noadjfile
disables the facilities provided by
.IR /etc/adjtime .
.B hwclock
will not read nor write to that file with this option. Either
.B \-\-utc
.B \-\-localtime
must be specified when using this option.
.BI \-\-adjfile= filename
overrides the default /etc/adjtime.
.BR \-f , \ \-\-rtc=\fIfilename\fB
overrides the default /dev file name, which is
.IR /dev/rtc
on many platforms but may be
.IR /dev/rtc0 ,
.IR /dev/rtc1 ,
and so on.
.B \-\-directisa
is meaningful only on an ISA machine or an Alpha (which implements enough
of ISA to be, roughly speaking, an ISA machine for
.BR hwclock 's
purposes). For other machines, it has no effect. This option tells
.B hwclock
to use explicit I/O instructions to access the Hardware Clock.
Without this option,
.B 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 read), it will
use the explicit I/O instructions anyway.
The rtc device driver was new in Linux Release 2.
.B \-\-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,
.B 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
.B \-\-badyear
if you have one of these machines. When
.B 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
.I hwclock \-\-set
.I hwclock \-\-systohc
at least once a year!
.B 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.
.B hwclock
warns you that you probably need
.B \-\-badyear
whenever it finds your Hardware Clock set to 1994 or 1995.
.B \-\-srm
This option is equivalent to
.B \-\-epoch=1900
and is used to specify the most common epoch on Alphas
with SRM console.
.B \-\-arc
This option is equivalent to
.B \-\-epoch=1980
and is used to specify the most common epoch on Alphas
with ARC console (but Ruffians have epoch 1900).
.B \-\-jensen
.B \-\-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
.B hwclock
should be able to determine by itself what it's
running on, at least when
.I /proc
is mounted.
(If you find you need one of these options to make
.B 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.)
.B \-\-jensen
means you are running on a Jensen model.
.B \-\-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.
.B \-\-test
Do everything except actually updating the Hardware Clock or anything
else. This is useful, especially in conjunction with
.BR \-\-debug ,
in learning about
.BR hwclock .
.B \-\-debug
Display a lot of information about what
.B hwclock
is doing internally. Some of its function is complex and this output
can help you understand how the program works.
.SH Clocks in a Linux System
There are two main clocks in a Linux system:
.B The Hardware Clock:
This is a clock that runs independently of any control program running
in the CPU and even when the machine is powered off.
On an ISA system, this clock is specified as part of the ISA standard.
The control program can read or set this clock to a whole second, but
the control program can also detect the edges of the 1 second clock
ticks, so the clock actually has virtually infinite precision.
This clock is commonly called the hardware clock, the real time clock,
the RTC, the BIOS clock, and the CMOS clock. Hardware Clock, in its
capitalized form, was coined for use by
.B hwclock
because all of the other names are inappropriate to the point of being
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.
.B 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
.BR 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
.BR 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
.I /usr/share/zoneinfo
directory, as explained in the man page for
.BR 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.
.B hwclock
sets the kernel timezone to the value indicated by TZ and/or
.I /usr/share/zoneinfo
when you set the System Time using the
.B \-\-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
.BR settimeofday (2).)
.SH How hwclock Accesses the Hardware Clock
.B 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
.I /dev/rtc0
.I /dev/rtc1
On older systems, the method of accessing the Hardware Clock depends on
the system hardware.
On an ISA system,
.B 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
.B 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,
.B hwclock
can access the clock via the console driver, via the device special
file /dev/tty1.
.B 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)
.B hwclock
will fall back to another method, if available. On an ISA or Alpha
machine, you can force
.B hwclock
to use the direct manipulation of the CMOS registers without even trying
.I /dev/rtc
by specifying the
.B \-\-directisa
.SH 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.
.BR hwclock 's
"adjust" function lets you make systematic corrections to correct the
systematic drift.
It works like this:
.B hwclock
keeps a file,
.IR /etc/adjtime ,
that keeps some historical information. This is called the adjtime file.
Suppose you start with no adjtime file. You issue a
.I hwclock \-\-set
command to set the Hardware Clock to the true current time.
.B 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
.I hwclock \-\-set
command to set it back 10 seconds.
.B 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
.I hwclock \-\-adjust
.B 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
.IR "hwclock \-\-adjust" .
.B 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
.I \-\-set
.IR \-\-systohc ),
.B 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
.B 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
.B hwclock
will do the adjustment then.
It is good to do a
.I hwclock \-\-adjust
just before the
.I 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
.BR 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
.B hwclock
command line.
You can use an adjtime file that was previously used with the
.BR clock (8)
program with
.BR hwclock .
.SH "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
.IR "hwclock \-\-hctosys" ,
that sets the System Time the old fashioned way.
To see if it is on or
off, use the command
.I 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
.I hwclock \-\-adjust
.IR "hwclock \-\-hctosys" .
You'll just make a mess. It is acceptable to use a
.I 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.
.SH 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.
.B 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
If you have a bona fide use for a CMOS century byte, contact the
.B 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.
.I /etc/adjtime
.I /usr/share/zoneinfo/
.RI ( /usr/lib/zoneinfo
on old systems)
.I /dev/rtc
.I /dev/rtc0
.I /dev/port
.I /dev/tty1
.I /proc/cpuinfo
.BR adjtimex (8),
.BR date (1),
.BR gettimeofday (2),
.BR settimeofday (2),
.BR crontab (1),
.BR tzset (3)
Written by Bryan Henderson, September 1996 (,
based on work done on the
.I 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-ng package and is available from