.TH CLOCK 8 "23 September 1996"
.SH NAME
clock \- query and set the ISA hardware clock (RTC)
.SH SYNOPSIS
.B "hwclock --show [ --utc ] [ --test ] [ --debug ]"
.br
.B "hwclock --set --date=newdate [ --utc ] [ --test ] [ --debug ]"
.br
.B "hwclock --systohc [ --utc ] [ --test ] [ --debug ]"
.br
.B "hwclock --hctosys [ --utc ] [ --test ] [ --debug ]"
.br
.B "hwclock --adjust [ --utc ] [ --test ] [ --debug ]"
.br
.B "hwclock --version [ --debug ]
.PP
Minimum unique abbreviations of all options are acceptable.
.PP
Also, equivalent options -r, -w, -s, -a, -v, -u, and -D are accepted for
compatibility with the program "clock".
.SH DESCRIPTION
.I 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.
.PP
You can also run
.I 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).
.SH OPTIONS
You need exactly one of the following options to tell
.I hwclock
what function to perform:
.PP
.TP
.B \-\-show
Read the Hardware Clock and print the time on Standard Output.
.TP
.B \-\-set
Set the Hardware Clock to the time given by the
.B \-\-date
option.
.TP
.B \-\-hctosys
Set the System Time from the Hardware Clock. This is a good option to use
in one of the system startup scripts.
.TP
.B \-\-systohc
Set the Hardware Clock to the current System Time.
.TP
.B \-\-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.
.TP
.B \-\-version
Print the version of
.I hwclock
on Standard Output.
.br
You need the following option if you specify
.B \-\-set
option. Otherwise, it is ignored.
.TP
.B \-\-date=date_string
Specifies the time to which to set the Hardware Clock. The value of this
option is an argument to the
.I date(1)
program. For example,
.sp
.I hwclock --set --date="9/22/96 16:45:05"
.PP
The following options apply to most functions.
.TP
.B \-\-utc
Indicates that the Hardware Clock is kept in Universal Coordinated
Time. 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
.I hwclock.
.PP
If you don't specify
.B --utc
when you should, or vice versa, both setting and querying of the
Hardware Clock will be messed up.
.TP
.B \-\-test
Do everything except actually updating the Hardware Clock. This is
useful, especially in conjunction with
.B \-\-debug,
in learning about
.I hwclock.
.TP
.B \-\-debug
Display a lot of information about what
.I hwclock
is doing internally. Some of its function is complex and this output
can help you understand how the program works.
.SH NOTES
.SH Clocks in a Linux System
.PP
There are two main clocks in a Linux system:
.PP
.B 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. 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.
.PP
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
.I hwclock
because all of the other names are inappropriate to the point of being
misleading.
.PP
.B 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.
.PP
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.
.PP
It is important that the System Time not have any discontinuities such as
would happen if you used the
.I 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
.I adjtimex(8)
to smoothly adjust the System Time while the system runs.
.SH The Adjust Function
.PP
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.
.I Hwclock's
"adjust" function lets you make systematic corrections to correct the
systematic drift.
.PP
It works like this:
.I Hwclock
keeps a file,
.I /etc/adjtime,
that keeps some historical information. This is called the adjtime file.
.PP
Suppose you start with no adjtime file. You issue a
.I hwclock --set
command to set the Hardware Clock to the true current time.
.I 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
.I hwclock --set
command to set it back 10 seconds.
.I 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
.I hwclock --adjust
command.
.I 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
.I hwclock --adjust.
.I 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.
.PP
Every time you calibrate (set) the clock,
.I 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.
.PP
A small amount of error creeps in any time
.I 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
.I hwclock
will do the adjustment then.
.PP
It is good to do a
.I hwclock --adjust
just before the
.I hwclock --set
at system startup time, and maybe periodically while the system is
running via cron.
.PP
The format of the adjtime file is:
.PP
Line 1: 3 numbers: 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
.I clock
).
.PP
Line 2: 1 number: Resulting number of seconds since 1969 UTC of most
recent calibration.
.PP
You can use an adjtime file that was previously used with the
.I clock
program with
.I hwclock.
.SH FILES
.I /etc/adjtime
.SH SEE ALSO
adjtimex(8), date(1), gettimeofday(2), settimeofday(2), crontab(1)
.SH AUTHORS
Written By Bryan Henderson, September 1996, based on work done on the
.I clock
program by Charles Hedrick, Rob Hooft, and Harald Koenig. See the source
code for complete history and credits.
