HWCLOCK(8) System Administration HWCLOCK(8)
NAME
hwclock - query or set the hardware clock (RTC)
SYNOPSIS
hwclock [function] [option...]
DESCRIPTION
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 sys‐
tematic drift (where the clock consistently loses or gains time at a certain rate when left to run).
FUNCTIONS
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 settime‐
ofday(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.
--systz
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 sys‐
tem 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.
--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 below.
--predict
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.
-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.
-h, --help
Display a help text and exit.
-V, --version
Display the version of hwclock and exit.
OPTIONS
The first two options apply to just a few specific functions, the others apply to most functions.
--date=date_string
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.
--epoch=year
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
--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 hwclock.
If you specify the wrong one of these options (or specify neither and take a wrong default), both set‐
ting 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
-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.
--directisa
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.
--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, 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.
--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).
--jensen
--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 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 auto‐
matically. Output of `hwclock --debug' and `cat /proc/cpuinfo' may be of interest.)
and this output can help you understand how the program works.
NOTES
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 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 run‐
ning 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 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 (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 Sys‐
tem Time using the --hctosys option.
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 run‐
ning 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 gener‐
ally 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 manipu‐
lation 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 com‐
mand 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 sec‐
onds 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.
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 Hard‐
ware 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 cen‐
tury 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 appro‐
priate.
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.
ENVIRONMENT VARIABLES
TZ
FILES
/etc/adjtime /usr/share/zoneinfo/ (/usr/lib/zoneinfo on old systems) /dev/rtc /dev/rtc0 /dev/port /dev/tty1
/proc/cpuinfo
nel.org/pub/linux/utils/util-linux/.
util-linux August 2011 HWCLOCK(8)
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