hwclock - query and set the hardware clock (RTC)
hwclock [functions] [options]
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
You can also run hwclock periodically to insert or remove time from the
Hardware Clock to compensate for systematic drift (where the clock con‐
sistently gains or loses time at a certain rate if left to run).
You need exactly one of the following options to tell hwclock what
function to perform:
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
--set Set the Hardware Clock to the time given by the --date option.
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/zone‐
info, 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
Set the Hardware Clock to the current System Time.
Set the kernel's timezone and reset the System Time based on the
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 ker‐
nel'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 reset.
Add or subtract time from the Hardware Clock to account for sys‐
tematic drift since the last time the clock was set or adjusted.
See discussion below.
Print the kernel's Hardware Clock epoch value to standard out‐
put. 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
Set the kernel's Hardware Clock epoch value to the value speci‐
fied by the --epoch option. See the --getepoch option for
Print the version of hwclock on Standard Output.
You need this option if you specify the --set option. Other‐
wise, it is ignored. This specifies the time to which to set
the Hardware Clock. The value of this option is an argument to
the date(1) program. For example,
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 --utc option.
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:
hwclock --setepoch --epoch=1952
The following options apply to most functions.
Indicates that the Hardware Clock is kept in Coordinated Univer‐
sal 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 nei‐
ther 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 local
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.
overrides the default /dev file name, which is /dev/rtc on many
platforms but may be /dev/rtc0, /dev/rtc1, and so on.
is meaningful only on an ISA machine or an Alpha (which imple‐
ments 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
read), it will use the explicit I/O instructions anyway.
The rtc device driver was new in Linux Release 2.
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 can‐
not 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
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 set‐
ting it, this scheme could be defeated and you could end up los‐
ing 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 deter‐
mine 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.)
--jensen means you are running on a Jensen model.
--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 inter‐
nally. Some of its function is complex and this output can help
you understand how the program works.
NOTESClocks 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 con‐
trol 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 exter‐
nal 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 ker‐
nel 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 sec‐
onds 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 Hard‐
ware 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 (per‐
haps 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_min‐
uteswest indicating how many minutes local time (not adjusted for DST)
lags behind UTC, and 2) a field tz_dsttime indicating the type of Day‐
light 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 speci‐
fying /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" regis‐
ters 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 instruc‐
tions, 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 rea‐
sons 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 --direc‐
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
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 com‐
mand to set the Hardware Clock to the true current time. Hwclock cre‐
ates 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 run‐
ning 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 inte‐
ger; 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 syn‐
chronized 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 Sys‐
tem 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 hardware.
/etc/adjtime /usr/share/zoneinfo/ (/usr/lib/zoneinfo on old systems)
/dev/rtc /dev/rtc0 /dev/port /dev/tty1 /proc/cpuinfo
SEE ALSOdate(1), gettimeofday(2), settimeofday(2), crontab(1), tzset(3)AUTHORS
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 cred‐
The hwclock command is part of the util-linux-ng package and is avail‐
able from ftp://ftp.kernel.org/pub/linux/utils/util-linux-ng/.
06 August 2008 HWCLOCK(8)