The Clock Mini-HOWTO
Ron Bean, v2.1, November 2000
How to set and keep your computer's clock on time.
- 1.1 Does Anybody Really Know What Time It Is?
- 1.2 Where to Find Stuff: "The Usual Places"
- 1.3 Acknowledgements
- 2.1 Basic Strategies
- 2.2 Potential Conflicts
- 2.3 Should the RTC use Local Time or UTC, and What About DST?
- 2.4 How Linux keeps Track of Time Zones
- 2.5 The Bottom Line
- 3.1 Clock(8) and Hwclock(8)
- 3.2 Adjtimex(8)
- 3.3 Xntpd and ntpd: the Network Time Protocol
- 3.4 Chrony
- 3.5 Clockspeed
- 4.1 CHU and the "Gadget Box"
- 4.2 WWV and the "Most Accurate Clock"
- 4.3 GPS and the "Totally Accurate Clock"
- 4.4 Low-frequency Time Signals: DCF77, MSF(Rugby), WWVB
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
- 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
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
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.
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
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
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
hwclock(8). The system clock
is corrected by adjusting the rate at which the system time is
advanced with each timer interrupt, using
A crude alternative to
adjtimex(8) is to have
hwclock(8) periodically to sync the system time to the
(corrected) RTC. This was recommended in the
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
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
ntpd as well), so if you're
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
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
adjtimex(2) (as used by
timed, for example) may turn this on. Calls to
settimeofday(2) will set
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
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
on older systems). The local time zone is determined by a symbolic
/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
This is nicely summarized at http://www.linuxsa.org.au/tips/time.html
If you don't need sub-second accuracy,
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
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
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
hwclock(8) for use in the
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
and you want to upgrade, you can find
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
hwclockI'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
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
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
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
hwclock(8), as stored in
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
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 "
adjtimex" (there is also an
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
Xntpd (NTPv3) has been replaced by
(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
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
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
Either list may or may not be up to date at any given time
:-). The list of drivers for
ntpd is at
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
You can get more information from Richard Curnow's website at
or http://go.to/chrony. There are
chrony mailing lists, one for announcements
and one for discussion by users. For information send email to
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 "
or by feeding the audio directly into the mic input of a Sun
SPARCstation (or any other machine with "compatible audio
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
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