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HWCLOCK(8) HWCLOCK(8)
NAME
hwclock - query and set the hardware clock (RTC)
SYNOPSIS
hwclock [functions] [options]
DESCRIPTION
hwclock is a tool for accessing the Hardware Clock. You can display the current time, set the Hardware Clock to a speci-
fied time, set the Hardware Clock to the System Time, and set the System Time from the Hardware Clock.
You can also run 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).
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.
--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 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.
--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
/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.
--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.
--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.
--setepoch
Set the kernel's Hardware Clock epoch value to the value specified by the --epoch option. See the --getepoch
option for details.
-v, --version
Print the version of hwclock on Standard Output.
--date=date_string
You need this option if you specify the --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 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.
--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:
hwclock --setepoch --epoch=1952
--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.
OPTIONS
The following options apply to most functions.
-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 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 time.
--noadjfile
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.
--adjfile=filename
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.
--directisa
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 read), it will
use the explicit I/O instructions anyway.
The rtc device driver was new in Linux Release 2.
--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 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 main-
tainer 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 con-
junction with --debug, in learning about hwclock.
--debug
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.
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 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. You can also
use the program 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 dis-
play a local time for you) almost always use a more traditional method of determining the timezone: They use the TZ envi-
ronment variable and/or the /usr/share/zoneinfo directory, as explained in the man page for tzset(3). However, some pro-
grams 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 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) conven-
tion 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 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 sup-
posed 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 adj-
time 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 sec-
onds per day as the systematic drift rate. 24 hours go by, and then you issue a hwclock --adjust command. Hwclock con-
sults 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 infor-
mation 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 cali-
bration 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 syn-
chronized 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.
To see if it is on or off, use the command 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 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.
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
SEE ALSO
adjtimex(8), date(1), gettimeofday(2), settimeofday(2), crontab(1), tzset(3)
AUTHORS
Written by Bryan Henderson, September 1996 (bryanhATgiraffe-data.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.
AVAILABILITY
The hwclock command is part of the util-linux-ng package and is available from ftp://ftp.kernel.org/pub/linux/utils/util-
linux-ng/.
06 August 2008 HWCLOCK(8)
