  The Clock Mini-HOWTO
  Ron Bean, rbean@execpc.com
  v2, July 1999

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

  Table of Contents


  1. Introduction

  2. Basic Timekeeping under Linux

  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

  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

  5. Detailed instructions for clock(8)

     5.1 Checking your installation
     5.2 Measuring your clock's drift rate
     5.3 Example
        5.3.1 To set time
        5.3.2 To reset time and check drift rate
        5.3.3 Calculating the correction factor


  ______________________________________________________________________

  1.  Introduction

  The Real-Time-Clock (RTC) chips used on PC motherboards (and even
  expensive workstations) are notoriously inaccurate, usually gaining or
  losing a consistent 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 "how to set up linux"
     documentation, and it can't be set up automatically without 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 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 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, but I've
  included detailed instructions for the old clock(8) program at the end
  for anyone still running an older system.


     Note
        You must be logged in as "root" to run any program that affects
        the RTC or the system time. Keep this in mind when setting up
        any of the programs described here.


     Note
        If you run more than one OS on your machine, you should only let
        one of them set the CMOS clock, so they don't confuse each
        other. This includes the twice-a-year adjustment for Daylight
        Savings Time.

  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.

  2.  Basic Timekeeping under Linux

  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. 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 the 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 in this case 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 have 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.

  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.

  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 radio clocks and
  such, 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 to have them automatically sync their clocks to each
  other.

  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 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.


     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.

  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.

  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
  <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.

  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
  <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.

  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-- all you need is 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").

  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
  <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.

  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
  <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>.

  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
  <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.

  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
  <http://www.arctime.com>

  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>.

  5.  Detailed instructions for clock(8)

  This section is retained from the earlier version of this mini-HOWTO,
  and is provided for anyone still using the old clock(8) program.
  Everything you need to know is in the man page, but the following
  discussion will walk you through the process.


     Note
        You must be root to run "clock", or any other program that
        affects either the system time or the CMOS clock.

  5.1.  Checking your installation

  Check your system startup files for a command like "clock -a" or
  "clock -ua".  Depending on which distribution you're using, it might
  be in /etc/rc.local, or /etc/rc.d/rc.sysinit, or some similar place.

  If it says "clock -s" or "clock -us", change the "s" to an "a", and
  then check to see if you have the file /etc/adjtime, which contains a
  single line that looks something like this:


       0.000000 842214901 0.000000

  These numbers are the correction factor (in seconds per day), the time
  the clock was last corrected (in seconds since Jan 1, 1970), and the
  partial second that was rounded off last time.  If you don't have this
  file, login as root and create it, with a single line that looks like
  this (all zeros):


       0.0 0 0.0

  Then run "clock -a" or "clock -ua" manually from the shell to update
  the 2nd number (use the "u" if your clock is set to Universal instead
  of local time).

  5.2.  Measuring your clock's drift rate

  First, you need to know what time it is :-). Your local time of day
  number may or may not be accurate. My favorite method is to call WWV's
  voice announcment at (303)499-7111 (this is a toll call). If you have
  access to a network time server, you can use the ntpdate program from
  the xntpd package (use the -b flag to keep the kernel from messing
  with the CMOS clock). Otherwise use "date -s hh:mm:ss" to set the
  kernel time by hand, and then "clock -w" to set the CMOS clock from
  the kernel clock. You'll need to remember when you last set the clock,
  so write down the date someplace where you won't lose it.  If you used
  ntpdate, do "date +%s" and write down the number of seconds since Jan
  1,1970.

  Then come back some days or weeks later and see how far the clock has
  drifted. If you're setting the clock by hand, I'd recommend waiting at
  least two weeks, and only calculate the drift rate to the nearest .1
  sec/day. After several months you could get to the nearest .01 sec/day
  (some people claim more accuracy than that, but I'm being conservative
  here). If you use ntpdate you shouldn't have to wait that long, but in
  any case you can always fine-tune it later.

  You can have cron run "clock -a" at regular intervals to keep the
  system time in line with the (corrected) CMOS time. This command will
  also be run from your startup file every time you boot the system, so
  if you do that often (as some of us do), that may be enough for your
  purposes.

  Note that certain programs may complain if the system time jumps by
  more than one second at a time, or if it jumps backwards.  If you have
  this problem, you can use xntpd or ntpdate to correct the time more
  gradually.

  5.3.  Example

  5.3.1.  To set time

  Login as root.  Dial (303)499-7111 (voice), listen for time
  announcement.  Then type:


       date -s hh:mm:ss

  but don't press enter until you hear the beep. (You could use "ntp­
  date" here instead of "date", and skip the phone call) This sets the
  "kernel time". Then type:

  clock -w

  This sets the CMOS time to match the kernel time.  Then type:


       date +%j

  (or "date +%s" if you used "ntpdate" instead of "date" above) and
  write down the number it gives you for next time.

  5.3.2.  To reset time and check drift rate

  Find the date you wrote down last time.  Login as root Then type:


       clock -a

  This sets the kernel time to match the current CMOS time.  Dial
  (303)499-7111 (voice), listen for announcement.  Then type:


       date

  and press enter when you hear the beep, but while you're waiting,
  write down the time they announce, and don't hang up yet.  This tells
  you what time your machine thought it was, when it should have been
  exactly on the minute.  Now type in


       date hh:mm:00

  using the minute *after* the one that was just announced, and press
  enter when you hear the beep again (now you can hang up).  For hh use
  the local hour.  This sets the "kernel time".  Then type:


       clock -w

  which writes the new (correct) time to the CMOS clock.  Now type:


       date +%j

  (or "date +%s" if that's what you used before)

  You now have three numbers (two dates and a time) that will allow you
  to calculate the drift rate.

  5.3.3.  Calculating the correction factor

  When you ran "date" on the minute, was your machine slow or fast? If
  it was fast, you'll have to subtract some number of seconds, so write
  it down as a negative number. If it was slow, you have to add some
  seconds, so write it down as positive.

  Now subtract the two dates. If you used "date +%j", the numbers
  represent the day-of-year (1-365, or 1-366 in leap years). If you've
  passed Jan 1 since you last set the clock you'll have to add 365 (or
  366) to the 2nd number. If you used "date +%s" then your number is in
  seconds, and you'll have to divide it by 86400 to get days.

  If you already had a correction factor in /etc/adjtime, you'll have to
  account for the number of seconds you've already corrected. If you've
  overcorrected, this number will have the opposite sign of the one you
  just measured; if you've undercorrected it will have the same sign.
  Multiply the old correction factor by the number of days, and then add
  the new number of seconds (signed addition-- if the two numbers have
  the same sign, you'll get a larger number, if they have opposite
  signs, you'll get a smaller number).

  Then divide the total number of seconds by the number of days to get
  the new correction factor, and put it in /etc/adjtime in place of the
  old one. Write down the new date (in seconds or days) for next time.

  Here's what my /etc/adjtime looks like:


       -9.600000 845082716 -0.250655

  (note 9.6 seconds per day is nearly five minutes per month!)

