All linux distributions install either the old clock(8)
or the newer hwclock(8), but without a correction
factor. Some may also install adjtimex(8), or they may
include it on the CD as optional (or you can download it from the
usual places). Some distributions also include a graphical clock
setting program that runs in an X-window, but they're designed
for interactive use and the system will still install
clock(8) or hwclock(8) for use in the startup
scripts.
Clock(8) requires you to calculate the correction factor
by hand, but hwclock(8) calculates it automatically
whenever you use it to reset the clock (using another program to
set the time will interfere with the drift correction, so always
use the same program if you're using a correction factor). If you
have an older system that still uses clock(8) and you
want to upgrade, you can find hwclock(8) in the
"util-linux" package, version 2.7 or later.
See the man page for more information.
The man page for hwclock(8) may be called
"clock" for backward compatibility, so
try both names. Hwclock(8) will respond to commands
written for clock(8), but the result may not be the
same-- in particular, "hwclock -a" is
not quite the same as "clock -a", so if
you're upgrading to hwclock I'd suggest replacing
all references to "clock" in your
startup scripts to use hwclock's native commands
instead.
The startup scripts vary from one distribution to another, so you
may have to search a bit to find where it sets the clock. Typical
locations are /etc/rc.local,
/etc/rc.d/rc.sysinit,
/etc/rc.d/boot, or some
similar place. The correction factor for the RTC is stored in
/etc/adjtime.
When you're setting the clock to determine the drift rate, keep in mind that your local telephone time announcement may or may not be accurate. If you don't have a shortwave radio or GPS receiver, you can hear the audio feed from WWV by calling (303)499-7111 (this is a toll call to Boulder, Colorado). It will cut you off after three minutes, but that should be long enough to set the clock. USNO and Canada's CHU also have telephone time services, but I prefer WWV's because there is more time between the announcement and the "beep".
If you get bizarre results from the RTC you may have a hardware problem. Some RTC chips include a lithium battery that can run down, and some motherboards have an option for an external battery (be sure the jumper is set correctly). The same battery maintains the CMOS RAM, but the clock takes more power and is likely to fail first. Bizarre results from the system clock may mean there is a problem with interrupts.
Adjtimex(8) allows the user to adjust the kernel's time
variables, and therefore change the speed of the system clock
(you must be logged in as "root" to do
this). It is cleverly designed to compare the system clock to the
RTC using the same correction factor used by clock(8) or
hwclock(8), as stored in /etc/adjtime.
So, once you've established the drift rate of the RTC, it's fairly simple
to correct the system clock as well. Once you've got it running
at the right speed, you can add a line to your startup scripts to
set the correct kernel variables at boot time. Since
adjtimex(8) was designed to work with clock(8)
or hwclock(8), it includes a work-around for the
"every 11 minutes" bug.
After you've installed adjtimex(8) you can get more
information on setting it up by typing "man 8
adjtimex" (there is also an adjtimex(2) man
page, which is not what you want) and by reading the
README file in /usr/doc/adjtimex-1.3/README
(where the version number in the path will be the current version
of adjtimex(8)).
Xntpd (NTPv3) has been replaced by ntpd
(NTPv4); the earlier version is no longer being maintained.
Ntpd is the standard program for synchronizing clocks
across a network, and it comes with a list of public time servers
you can connect to. It can be a little more complicated to set up
than the other programs described here, but if you're interested
in this kind of thing I highly recommend that you take a look at
it anyway. The "home base" for information on ntpd
is the NTP website at
http://www.eecis.udel.edu/~ntp/
which also includes links to all kinds of interesting time-related
stuff (including software for other OS's). Some linux
distributions include ntpd on the CD.
A relatively new feature in ntpd is a "burst mode"
which is designed for machines that have only intermittent
dial-up access to the internet.
Ntpd includes drivers for quite a few radio clocks
(although some appear to be better supported than others). Most
radio clocks are designed for commercial use and cost thousands
of dollars, but there are some cheaper alternatives (discussed in
later sections). In the past most were WWV or WWVB receivers, but
now most of them seem to be GPS receivers. NIST has a PDF file
that lists manufacturers of radio clocks on their website at
http://www.boulder.nist.gov/timefreq/links.htm (near
the bottom of the page). The NTP website also includes many links
to manufacturers of radio clocks at
http://www.eecis.udel.edu/~ntp/hardware.htm and
http://www.eecis.udel.edu/~mills/ntp/refclock.htm.
Either list may or may not be up to date at any given time :-).
The list of drivers for ntpd is at
http://www.eecis.udel.edu/~ntp/ntp_spool/html/refclock.htm.
Ntpd also includes drivers for several dial-up time
services. These are all long-distance (toll) calls, so be sure to
calculate the effect on your phone bill before using them.
Xntpd was originally written for machines that have a
full-time connection to a network time server or radio clock. In
theory it can also be used with machines that are only connected
intermittently, but Richard Curnow couldn't get it to work the
way he wanted it to, so he wrote "chrony" as an
alternative for those of us who only have network access when
we're dialed in to an ISP (this is the same problem that
ntpd's new "burst mode" was designed to solve).
The current version of chrony includes drift correction
for the RTC, for machines that are turned off for long periods of
time.
You can get more information from Richard Curnow's website at http://www.rrbcurnow.freenet.co.uk/chrony/index.html. He distributes the program as source code only, but Debian has been including a binary in their "unstable" collection. The source file is also available at the usual linux archive sites.