The Clock Mini-HOWTO
v2.1, November 2000
How to set and keep your computer's clock on time.
The Real-Time-Clock (RTC) chips used on PC motherboards are
notoriously inaccurate, usually gaining or losing the same
amount of time each day. Linux provides a simple way to correct
for this in software, which can make the clock *very*
accurate, even without an external time source. But most people
don't know how to set it up, for several reasons:
- It's not mentioned in most of the general documentation
on how to set up linux, and it can't be set up
automatically (unless you have an external time source),
so the default is not to use it.
- If you type "
man clock" you may get
the man page for clock(3), which is not what you
want. Try "man 8 clock" or
"man 8 hwclock" (some distributions
search the man pages in numerical order if you don't give
a section number, others search in the order specified in
/etc/man.config).
- Most people don't seem to care what time it is anyway.
- Those few who do care often want to sync the system clock
to an external time source, such as a network time server
or radio clock. This makes the accuracy of the RTC
(mostly) irrelevant.
This mini-HOWTO describes the low-tech approach (which can be
very accurate by itself), and provides pointers to several more
sophisticated options. In most cases the documentation is well
written, so I'm not going to repeat that information here.
Previous versions included detailed instructions for the old
clock(8) program for anyone still running an older
system, but I've dropped that section because most distributions
now use hwclock(8) instead, which has much better
documentation. If you still want a copy of the clock(8)
instructions I can email them to you, but read the section on
hwclock(8) first.
- Note
You must be logged in as "root" to run
any program that affects the RTC or the system time, which
includes most of the programs described here. If you
normally use a graphical interface for everything, you may
also need to learn some basic unix shell commands.
- Note
If you run more than one OS on your machine, you should
only let one of them set the RTC, so they don't
confuse each other. The exception is the twice-a-year
adjustment for Daylight Saving(s) Time (see the section on
DST for details).
If you run a dual-boot system that spends a lot of time running
Windows, you may want to check out some of the clock software
available for that OS instead. Follow the links on the NTP
website at
http://www.eecis.udel.edu/~ntp/software.html.
Many of the radio clocks mentioned here include software for Windows.
In some places I've mentioned that software can be downloaded
from "the usual places", which means any place you
could download a complete Linux system if you didn't get it on a
CD-ROM. In the old days that meant the ftp archive at
sunsite.unc.edu, and various mirror sites around the world. That
site has been renamed
http://metalab.unc.edu/linux/
(since Sun no longer sponsors it). Some distributions also have
their own websites, which may include a lot of this stuff.
I assume most people get Linux on CD these days, and those CDs
often include software that is not part of the default
installation, so you may already have some of the programs
mentioned here without knowing it.
The latest version of this mini-HOWTO can be found at the home of
the Linux Documentation Project, which is currently
http://www.linuxdoc.org/ (and is also reachable from
the metalab site mentioned above). I think all the old links are
now forwarded to this one.
All HOWTOs are written in SGML and converted to various other
formats by standardized conversion programs. Most people seem to
want the HTML version, which is at
http://www.linuxdoc.org/HOWTO/mini/Clock.html.
Revision history can be found as comments in the SGML source.
Most Linux distributions install a complete set of HOWTO's in
/usr/doc/HOWTO/ and
/usr/doc/HOWTO/mini.
This mini-HOWTO has been greatly improved thanks to various
people who have sent me email since the first version in 1996.
In some cases they wrote with questions but ended up giving me as
much information as I gave them. Unfortunately I haven't compiled
a list of names (maybe next time). You know who you are :-).
A Linux system actually has two clocks: One is the battery
powered "Real Time Clock" (also known as the
"RTC", "CMOS clock", or "Hardware
clock") which keeps track of time when the system is turned
off but is not used when the system is running. The other is the
"system clock" (sometimes called the "kernel
clock" or "software clock") which is a software
counter based on the timer interrupt. It does not exist when the
system is not running, so it has to be initialized from the RTC
(or some other time source) at boot time. References to
"the clock" in the ntpd documentation refer
to the system clock, not the RTC.
The two clocks will drift at different rates, so they will
gradually drift apart from each other, and also away from the
"real" time. The simplest way to keep them on time is to
measure their drift rates and apply correction factors in
software. Since the RTC is only used when the system is not
running, the correction factor is applied when the clock is
read at boot time, using clock(8) or hwclock(8).
The system clock is corrected by adjusting the rate at which the system
time is advanced with each timer interrupt, using adjtimex(8).
A crude alternative to adjtimex(8) is to have
chron run clock(8) or hwclock(8)
periodically to sync the system time to the (corrected) RTC. This
was recommended in the clock(8) man page, and it works
if you do it often enough that you don't cause large
"jumps" in the system time, but adjtimex(8)
is a more elegant solution. Some applications may complain if the
time jumps backwards.
The next step up in accuracy is to use a program like
ntpd to read the time periodically from a network time
server or radio clock, and continuously adjust the rate of the
system clock so that the times always match, without causing
sudden "jumps" in the system time. If you always have a
network connection at boot time, you can ignore the RTC
completely and use ntpdate (which comes with the
ntpd package) to initialize the system clock from a
time server-- either a local server on a LAN, or a remote server
on the internet. But if you sometimes don't have a network
connection, or if you need the time to be accurate during the
boot sequence before the network is active, then you need to
maintain the time in the RTC as well.
It might seem obvious that if you're using a program like
ntpd, you would want to sync the RTC to the (corrected)
system clock. But this turns out to be a bad idea if the system
is going to stay shut down longer than a few minutes, because it
interferes with the programs that apply the correction factor to
the RTC at boot time.
If the system runs 24/7 and is always rebooted immediately
whenever it's shut down, then you can just set the RTC from the
system clock right before you reboot. The RTC won't drift enough
to make a difference in the time it takes to reboot, so you don't
need to know its drift rate.
Of course the system may go down unexpectedly, so some versions
of the kernel sync the RTC to the system clock every 11 minutes
if the system clock has been adjusted by another program. The RTC
won't drift enough in 11 minutes to make any difference, but if
the system is down long enough for the RTC to drift
significantly, then you have a problem: the programs that apply
the drift correction to the RTC need to know *exactly*
when it was last reset, and the kernel doesn't record that
information anywhere.
Some unix "traditionalists" might wonder why anyone
would run a linux system less than 24/7, but some of us run
dual-boot systems with another OS running some of the time, or
run Linux on laptops that have to be shut down to conserve
battery power when they're not being used. Other people just
don't like to leave machines running unattended for long periods
of time (even though we've heard all the arguments in favor of
it). So the "every 11 minutes" feature becomes a bug.
This "feature/bug" appears to behave differently in
different versions of the kernel (and possibly in different
versions of xntpd and ntpd as well), so if
you're running both ntpd and hwclock you may
need to test your system to see what it actually does. If you
can't keep the kernel from resetting the RTC, you might have to
run without a correction factor on the RTC.
The part of the kernel that controls this can be found in
/usr/src/linux-2.0.34/arch/i386/kernel/time.c
(where the version number in the path will be the version of the
kernel you're running). If the variable time_status is
set to TIME_OK then the kernel will write the system
time to the RTC every 11 minutes, otherwise it leaves the RTC
alone. Calls to adjtimex(2) (as used by ntpd
and timed, for example) may turn this on. Calls to
settimeofday(2) will set time_status to
TIME_UNSYNC, which tells the kernel not to adjust the
RTC. I have not found any real documentation on this.
I've heard reports that some versions of the kernel may have
problems with "sleep modes" that shut down the CPU to
save energy. The best solution is to keep your kernel up to date,
and refer any problems to the people who maintain the kernel.
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.
The Linux "system clock" actually just counts the
number of seconds past Jan 1, 1970, and is always in UTC (or GMT,
which is technically different but close enough that casual users
tend to use both terms interchangeably). UTC does not change as
DST comes and goes-- what changes is the conversion between UTC
and local time. The translation to local time is done by
library functions that are linked into the application programs.
This has two consequences: First, any application that needs
to know the local time also needs to know what time zone you're
in, and whether DST is in effect or not (see the next section for
more on time zones). Second, there is no provision in the kernel to
change either the system clock or the RTC as DST comes and goes,
because UTC doesn't change. Therefore, machines that only run
Linux should have the RTC set to UTC, not local time.
However, many people run dual-boot systems with other OS's that
expect the RTC to contain the local time, so hwclock
needs to know whether your RTC is in local time or UTC, which it
then converts to seconds past Jan 1, 1970 (UTC). This still does
not provide for seasonal changes to the RTC, so the change must
be made by the other OS (this is the one exception to the rule
against letting more than one program change the time in the RTC).
Unfortunately, there are no flags in the RTC or the CMOS RAM to
indicate standard time vs DST, so each OS stores this information
someplace where the other OS's can't find it. This means that
hwclock must assume that the RTC always contains the
correct local time, even if the other OS has not been run since
the most recent seasonal time change.
If Linux is running when the seasonal time change occurs, the
system clock is unaffected and applications will make the
correct conversion. But if linux has to be rebooted for any
reason, the system clock will be set to the time in the RTC,
which will be off by one hour until the other OS (usually
Windows) has a chance to run.
There is no way around this, but Linux doesn't crash very often,
so the most likely reason to reboot on a dual-boot system is to
run the other OS anyway. But beware if you're one of those people
who shuts down Linux whenever you won't be using it for a while--
if you haven't had a chance to run the other OS since the last
time change, the RTC will be off by an hour until you do.
Some other documents have stated that setting the RTC to UTC
allows Linux to take care of DST properly. This is not really
wrong, but it doesn't tell the whole story-- as long as you don't
reboot, it does not matter which time is in the RTC (or even if
the RTC's battery dies). Linux will maintain the correct time
either way, until the next reboot. In theory, if you only reboot
once a year (which is not unreasonable for Linux), DST could come
and go and you'd never notice that the RTC had been wrong for
several months, because the system clock would have stayed
correct all along. But since you can't predict when you'll want
to reboot, it's better to have the RTC set to UTC if you're not
running another OS that requires local time.
The Dallas Semiconductor RTC chip (which is a drop-in replacement
for the Motorola chip used in the IBM AT and clones) actually has
the ability to do the DST conversion by itself, but this feature
is not used because the changeover dates are hard-wired into the
chip and can't be changed. Current versions change on the first
Sunday in April and the last Sunday in October, but earlier
versions used different dates (and obviously this doesn't work in
countries that use other dates). Also, the RTC is often
integrated into the motherboard's "chipset" (rather
than being a separate chip) and I don't know if they all have
this ability.
You probably set your time zone correctly when you
installed Linux. But if you have to change it for some reason, or
if the local laws regarding DST have changed (as they do
frequently in some countries), then you'll need to know how to
change it. If your system time is off by some exact number of
hours, you may have a time zone problem (or a DST problem).
Time zone and DST information is stored in
/usr/share/zoneinfo (or
/usr/lib/zoneinfo on older systems).
The local time zone is determined by a symbolic link from
/etc/localtime to one of these files.
The way to change your timezone is to change the link.
If your local DST dates have changed, you'll have to edit the
file.
You can also use the TZ environment variable to change
the current time zone, which is handy of you're logged in
remotely to a machine in another time zone. Also see the man
pages for tzset and tzfile.
This is nicely summarized at
http://www.linuxsa.org.au/tips/time.html
If you don't need sub-second accuracy, hwclock(8) and
adjtimex(8) may be all you need. It's easy to get
enthused about time servers and radio clocks and so on, but I ran
the old clock(8) program for years with excellent results.
On the other hand, if you have several machines on a LAN it can be
handy (and sometimes essential) to have them automatically sync
their clocks to each other. And the other stuff can be fun to
play with even if you don't really need it.
On machines that only run Linux, set the RTC to UTC (or GMT).
On dual-boot systems that require local time in the RTC, be aware
that if you have to reboot Linux after the seasonal time change,
the clock may be temporarily off by one hour, until you have a
chance to run the other OS. If you run more than two OS's, be
sure only one of them is trying to adjust for DST.
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 Linux archive sites). Some distributions also include
a graphical clock setting program that runs in an X-window, but
those are 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 RTC (using another program to
set the RTC 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.
- Note
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.
Red Hat has a script in
/etc/sysconfig/clock that controls the
options to hwclock.
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".
You can also get the time from a network time server using the
ntpdate program that comes with ntpd, and
there's a javaclock at
www.time.gov.
In any case, what you're setting is the system clock, not the RTC
(see the man page for the date command for the formats
to use). Then use hwclock to set the RTC and calculate
the drift rate. If you're doing this by hand, you should be able
to set it within a second or two, and get a reasonable
approximation of the drift rate after a few weeks. Then you can
run adjtimex(8) to fine-tune the system clock.
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. There is a list of
public time servers at
http://www.eecis.udel.edu/~mills/ntp/clock2.html.
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.freeuk.com/chrony or
http://go.to/chrony.
There are also two chrony mailing lists, one for
announcements and one for discussion by users. For information
send email to
chrony-users-subscribe@egroups.com
or
chrony-announce-subscribe@egroups.com
Chrony is normally distributed 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.
Another option is the clockspeed program by DJ
Bernstein. It gets the time from a network time server and simply
resets the system clock every three seconds. It can also be used
to synchronize several machines on a LAN.
I've sometimes had trouble reaching his website at
http://Cr.yp.to/clockspeed.html, so if you get a DNS
error try again on another day. I'll try to update this
section if I get some better information.
CHU, the Canadian shortwave time station near Ottawa, is similar
to WWV in the US but with one important difference: in addition
to announcing the time in both French and English, it also
broadcasts the current time once per minute using the old "Bell
103" (300 baud) modem tones. These tones are very easy to
decode, and Bill Rossi realised that you don't even need a
modem-- just a shortwave radio and a sound card. If you're able
to receive the signal from CHU, this may be the cheapest radio
clock available. Shortwave reception varies throughout the day,
but Bill claims that by changing frequencies twice a day (morning
and evening) he gets almost 24-hour coverage. CHU broadcasts on
3.33, 7.335, and 14.670 MHz.
For more information see Bill Rossi's website at
http://www.rossi.com/chu/. The source file is also
available at the usual Linux archive sites. For information on
CHU's time services see
http://www.nrc.ca/inms/time/ctse.html.
The NTP website has plans for a "gadget box" that
decodes the CHU time broadcast using an inexpensive 300 baud
modem chip and any shortwave radio, at
http://www.eecis.udel.edu/~ntp/ntp_spool/html/gadget.htm.
The plans include a Postscript image of a
2-sided custom printed circuit board, but you have to make the
board yourself (or find someone who can make it for you).
Ntpd includes a driver (type 7) for CHU receivers, which
works either with modems like the "gadget box",
or by feeding the audio directly into the mic input of a Sun SPARCstation
(or any other machine with "compatible audio drivers").
You may have heard about Heathkit's "Most Accurate Clock",
which received and decoded the time signal from WWV and had an
optional serial port for connecting to a computer. Heathkit
stopped selling kits a long time ago, but they continued to sell
the factory-built version of the clock until 1995, when it was
also discontinued. For Heathkit nostalgia (not including the
clock) see
http://www.heathkit-museum.com. The
Heathkit company still exists, selling educational materials. See
http://www.heathkit.com.
According to Dave Mills, Heathkit's patent on the "Most Accurate
Clock" is due to expire soon, so maybe someone out there would
like to clone it as a single-chip IC.
The NTP website has a DSP program (and a PDF file describing it) at
http://www.eecis.udel.edu/~mills/resource.htm
that decodes the WWV time signal using a shortwave radio and the
TAPR/AMSAT DSP-93, a DSP kit which is no longer available. It was
based on the Texas Instruments TMS320C25 DSP chip. The TAPR
website at
http://www.tapr.org includes a lot of
information on homebrew DSP programming.
Ntpd includes a driver (type 6) for the IRIG-B and
IRIG-E time codes, using /dev/audio on a
Sun SPARCstation, with a note that it is "likely portable
to other systems". WWV uses the IRIG-H time code.
WWV is run by NIST, which has a website at
http://www.boulder.nist.gov/timefreq/index.html.
This site includes the text of "Special Publication 432",
which describes their time and frequency services, at
http://www.boulder.nist.gov/timefreq/pubs/sp432/sp432.htm.
WWV broadcasts on 2.5, 5, 10, 15, and 20 Mhz.
GPS signals include the correct time, and some GPS receivers have
serial ports. Ntpd includes drivers for several GPS
receivers. The 1PPS feature ("One Pulse Per Second",
required for high accuracy) usually requires a separate interface
to connect it to the computer.
TAPR (Tuscon Amateur Packet Radio) makes a kit for an interface
called "TAC-2" (for "Totally Accurate
Clock") that plugs into a serial port and works with any
GPS receiver that can provide a 1PPS output-- including some
"bare board" models that can be mounted directly to
the circuit board. For more information see their website at
http://www.tapr.org. The price (as of June 1999) is around
$140, not including the GPS receiver. The kit does not
include any enclosure or mounting hardware.
The CHU "gadget box" (described in another section)
can also be used as an interface for the 1PPS signal. The NTP website
has a discussion of this at
http://www.eecis.udel.edu/~ntp/ntp_spool/html/pps.htm.
These low-frequency stations broadcast a time code by simply
switching the carrier on and off. Each station uses its own
coding scheme, and summaries are available on the NTP website at
http://www.eecis.udel.edu/~mills/ntp/index.htm
(near the bottom of the page). DCF77 in Germany broadcasts on
77.5kHz. MSF in England (also called "Rugby", which
apparently refers to its location) and WWVB in Colorado both
broadcast on 60 kHz.
Reception of WWVB varies, but there are plans to increase its
broadcast power, in several stages. You can follow its progress
on NIST's website at
http://www.boulder.nist.gov/timefreq/wwvstatus.html.
Inexpensive receivers that can plug into a serial port are
reported to be available in Europe. Ntpd includes
drivers for a couple of MSF receivers.
A number of companies in the US sell relatively inexpensive
clocks that have built-in WWVB receivers (including several
analog wall clocks), but I'm only aware of two that can be
connected to a computer:
The Ultralink Model 320 sells for about $120 (as of June
1999) and has a serial interface and a straightforward ASCII
command set, so it shouldn't be too hard to program. It draws 1mA
from the serial port for power. The antenna can be up to 100 feet
away from the computer, and the unit contains its own clock to
maintain the time if it loses the signal. They also sell a
"bare board" version for about $80 that is
designed to work with the "BASIC Stamp" series of
microcontrollers. See
http://www.ulio.com/timepr.html.
Arcron Technology sells a desk clock with an optional serial
port for about $130, including software for Windows. See
