CLOCK(8)
NAME
clock - query and set the ISA hardware clock (RTC)
SYNOPSIS
hwclock --show [ --utc ] [ --test ] [ --debug ]
hwclock --set --date=newdate [ --utc ] [ --test ] [
--debug ]
hwclock --systohc [ --utc ] [ --test ] [ --debug ]
hwclock --hctosys [ --utc ] [ --test ] [ --debug ]
hwclock --adjust [ --utc ] [ --test ] [ --debug ]
hwclock --version [ --debug ]
Minimum unique abbreviations of all options are accept-
able.
Also, equivalent options -r, -w, -s, -a, -v, -u, and -D
are accepted for compatibility with the program "clock".
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 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).
OPTIONS
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 Stan-
dard Output.
--set Set the Hardware Clock to the time given by the
--date option.
--hctosys
Set the System Time from the Hardware Clock. This
is a good option to use in one of the system
startup scripts.
--systohc
Set the Hardware Clock to the current System 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.
--version
Print the version of hwclock on Standard Output.
You need the following option if you specify --set
option. Otherwise, it is ignored.
--date=date_string
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 following options apply to most functions.
--utc Indicates that the Hardware Clock is kept in Uni-
versal Coordinated Time. It is your choice whether
to keep your clock in UTC or local time, but noth-
ing in the clock tells which you've chosen. So
this option is how you give that information to
hwclock.
If you don't specify --utc when you should, or vice versa,
both setting and querying of the Hardware Clock will be
messed up.
--test Do everything except actually updating the Hardware
Clock. This is useful, especially in conjunction
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 indepen-
dently of any control program running in the CPU and even
when the machine is powered off. It 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 inappro-
priate to the point of being misleading.
The System Time: This is the time kept by a clock inside
the Linux kernel and driven by the ISA timer interrupt.
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 discon-
tinuities 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.
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 command to set
it back 10 seconds. Hwclock updates the adjtime file to
show the current time as the last time the clock was cali-
brated, 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, hwclock recalcu-
lates 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 --set at system startup time, and maybe periodi-
cally while the system is running via cron.
The format of the adjtime file is:
Line 1: 3 numbers: 1) systematic drift rate in seconds per
day, floating point decimal; 2) Resulting number of sec-
onds since 1969 UTC of most recent adjustment or calibra-
tion, decimal integer; 3) zero (for compatibility with
clock ).
Line 2: 1 number: Resulting number of seconds since 1969
UTC of most recent calibration.
You can use an adjtime file that was previously used with
the clock program with hwclock.
FILES
/etc/adjtime
SEE ALSO
adjtimex(8) date(1) gettimeofday(2) settimeofday(2)
crontab(1)
AUTHORS
Written By Bryan Henderson, September 1996, based on work
done on the clock program by Charles Hedrick, Rob Hooft,
and Harald Koenig. See the source code for complete his-
tory and credits.