hwclock - query and set the hardware clock (RTC)
hwclock -r or hwclock --show
hwclock -w or hwclock --systohc
hwclock -s or hwclock --hctosys
hwclock -a or hwclock --adjust
hwclock -v or hwclock --version
hwclock --set --date=newdate
hwclock --setepoch --epoch=year
[-u|--utc] --localtime --noadjfile --directisa --test [-D|--debug]
and arcane options for DEC Alpha:
[-A|--arc] [-J|--jensen] [-S|--srm] [-F|--funky-toy]
Minimum unique abbreviations of all options are acceptable.
Also, -h asks for a help message.
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 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 hwclock what function to perform:
--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.
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.
Set the Hardware Clock to the current System Time.
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.
Print out standard output the kernel's Hardware Clock epoch value. 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.
Set the kernel's Hardware Clock epoch value to the value specified by the --epoch
option. See the --getepoch option for details.
Print the version of hwclock on Standard Output.
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.
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 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 speci-
fied 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.
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.
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
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 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 cal-
ibrated 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, which-
ever 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).
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 automat-
ically. 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 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 pro-
gram 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 preci-
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.
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 preci-
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
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 mis-
led -- 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/local/timezone 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 time-
stamps on files.
hwclock sets the kernel timezone to the value indicated by TZ and/or /usr/local/timezone
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_dst-
time 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 hang-
On older systems, the method of accessing the Hardware Clock depends on the system hard-
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 pro-
vides 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, hwclock will fall back to another method, if
available. On an ISA or Alpha machine, you can force hwclock to use the direct manipula-
tion of the CMOS registers without even trying /dev/rtc by specifying the --directisa
The Adjust Function
The Hardware Clock is usually not very accurate. However, much of its inaccuracy is com-
pletely 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 infor-
mation. 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 command to set it back 10 sec-
onds. 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
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
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 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
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 Univer-
sal 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
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 sophisti-
cated 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.
To see if it is on or off, use the command adjtimex --print and look at the value of "sta-
tus". 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.
/etc/adjtime /usr/share/zoneinfo/ (/usr/lib/zoneinfo on old systems) /dev/rtc /dev/port
adjtimex(8), date(1), gettimeofday(2), settimeofday(2), crontab(1), tzset(3)
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.
02 March 1998 HWCLOCK(8)