hwclock - query or set the hardware clock (RTC)
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
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 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.
- Set the kernel's timezone and reset the System Time based on the current
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,
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.
- 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
- 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
- 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
- -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
- -h, --help
- Display help text and exit.
- -V, --version
- Display version information 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
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
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 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.
- This option is equivalent to --epoch=1900 and is used to specify
the most common epoch on Alphas with SRM console.
- 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
- 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
- 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 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
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.
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
directory, as explained in the man page for
. 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.
sets the kernel timezone to the value indicated by TZ and/or
when you set the System Time using the
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
Users 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 is restriction is not necessary if you haven't
installed setuid root, but it's there for now).
How hwclock Accesses the Hardware Clock¶
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
On older systems, the method of accessing the Hardware Clock depends on the
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.
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
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
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
to set the Hardware Clock to the true current time. Hwclock
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
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
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
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
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
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
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
. You'll just make a mess. It is acceptable to use a
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.
systems) /dev/rtc /dev/rtc0 /dev/port /dev/tty1
Written by Bryan Henderson, September 1996 (email@example.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