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  The Clock Mini-HOWTO
  Ron Bean,
  v2.1, November 2000

  How to set and keep your computer's clock on time.

  Table of Contents

  1. Introduction

     1.1 Does Anybody Really Know What Time It Is?
     1.2 Where to Find Stuff: "The Usual Places"
     1.3 Acknowledgements

  2. How Linux Keeps Track of Time

     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. Software

     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. Radio Clocks

     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


  1.  Introduction

  1.1.  Does Anybody Really Know What Time It Is?

  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.

        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

        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
  <>.  Many of the radio
  clocks mentioned here include software for Windows.

  1.2.  Where to Find Stuff: "The Usual Places"

  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, and various mirror
  sites around the world. That site has been renamed
  <> (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

  The latest version of this mini-HOWTO can be found at the home of the
  Linux Documentation Project, which is currently
  <> (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 <>.
  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.

  1.3.  Acknowledgements

  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 :-).

  2.  How Linux Keeps Track of Time

  2.1.  Basic Strategies

  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.

  2.2.  Potential Conflicts

  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

  2.3.  Should the RTC use Local Time or UTC, and What About DST?

  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

  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.

  2.4.  How Linux keeps Track of Time Zones

  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

  2.5.  The Bottom Line

  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.

  3.  Software

  3.1.  Clock(8) and Hwclock(8)

  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

  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

        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

  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  <>.

  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.

  3.2.  Adjtimex(8)

  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)).

  3.3.  Xntpd and ntpd: the Network Time Protocol

  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
  <> 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

  A relatively new feature in ntpd is a "burst mode" which is designed
  for machines that have only intermittent dial-up access to the

  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
  <> (near the bottom of
  the page). The NTP website also includes many links to manufacturers
  of radio clocks at <> and
  <>.  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.

  3.4.  Chrony

  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
  <> or <>.
  There are also two chrony mailing lists, one for announcements and one
  for discussion by users. For information send email to chrony-users- or

  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.

  3.5.  Clockspeed

  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
  <>, so if you get a DNS error try again
  on another day. I'll try to update this section if I get some better

  4.  Radio Clocks

  4.1.  CHU and the "Gadget Box"

  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
  <>. The source file is also available at the
  usual Linux archive sites. For information on CHU's time services see

  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 <>.
  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").

  4.2.  WWV and the "Most Accurate Clock"

  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
  <>. The Heathkit company still exists,
  selling educational materials. See <>.

  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
  <> 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
  <> includes a lot of information on homebrew DSP

  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
  <>.  This site includes
  the text of "Special Publication 432", which describes their time and
  frequency services, at
  <>.  WWV
  broadcasts on 2.5, 5, 10, 15, and 20 Mhz.

  4.3.  GPS and the "Totally Accurate Clock"

  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
  <>. 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 <>.

  4.4.  Low-frequency Time Signals: DCF77, MSF(Rugby), WWVB

  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
  <> (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

  Inexpensive receivers that can plug into a serial port are reported to
  be available in Europe. Ntpd includes drivers for a couple of MSF

  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

  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  <>.

  Arcron Technology sells a desk clock with an optional serial port for
  about $130, including software for Windows. See

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