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Debian GNU/Linux System Administrator's Manual (Obsolete Documentation)
Chapter 16 - Time


Computers have a clock to keep time. Usually there is a hardware clock with battery backup to keep time when the computer is off. The operating system (for example Linux) runs its own clock, and from this clock comes the time shown by commands such as date in Linux and time in DOS.

The hardware clock is usually accurate enough, provided that the battery has not run out. If the time shown by the computer when you turn it on is wrong by several months or years, it is worth checking if the battery is still usable.

The "software" clock in the operating system usually has drift, either systematic or random. This drift means the clock runs too fast or too slow. For this reason it is nesessary to use some accurate time source to syncronize the operating system clock if accurate time is needed.

To see the time on Debian GNU/Linux, use the command date. For example

     $ date
     Fri Oct 23 04:45:51 EEST 1998

The above command shows the day of the week, the month, the day of the month, the time, the time zone and the year. The time zone also shows whether Daylight Saving Time is in use (in the example, the base time zone is EET,and the extra S means Saving).

In Debian GNU/Linux and other Unix, the command time does not show time. It is used to time command excecutions. If you have some command that takes a fair amount of time to execute, and you do not feel like standing by with a stopwatch, you can see how long it took by running the command as argument to time. This example may clear things up:

     $ time sleep 60
     real    1m0.045s
     user    0m0.000s
     sys     0m0.000s

If You have root privileges, you can also change the time with the command date (see the man page date(1)). If you are connected to the Internet or have other Unix computers on your LAN, see Syncing time, rdate and NTP, Section 16.4 for easier ways to set time.

When setting the time manually, the time string may be confusing. The command date --set accepts the date and time in many formats. You can read the sh-utils info document, or use the example below to figure out one possible format. There date is given in ISO 8601 standard format YYYY-MM-DD for Year-Month-DayOfMonth, and time of day using 24 hour clock. Leading zeros are significant.

     date --set 1998-11-02 
     date --set 21:08:0

The above two commands set the date to 2nd November 1998, and time to eight minutes past 9pm.

To see the time in UTC, use command date -u.

If you are running X Window, you may have a clock display somewhere on the screen. If not, you can obtain it by running the command xclock, which is part of the xbase package. The bare xclock command gives you an analog clock (with big hand, little hand, and an optional seconds hand). You can get a digital clock (with xclock -digital). See the man page xclock(1)for more information.

16.1 Setting time, time zones and Daylight Saving

When you installed the base system of Debian GNU/Linux, you set the Timezone. You can check how you have set the Timezone by looking at the file /etc/timezone. If you want to change the timezone configuration, see Changing the timezone after installation, Section 16.1.1.

     $ cat /etc/timezone

The time zone is needed because Unix computers keep time in Universal Time (UTC), and local time is calculated from this. UTC is solar time on meridian 0. UTC was previously called Greenwich Mean Time (GMT) because meridian 0 passes through the old Royal Observatory in Greenwich, which is part of London, England.

UTC is constant, and is not subject to Daylight Saving Time or other changes. This is what makes it useful for syncronising computers. As long as the base time is kept in UTC, computers all over the world can be synchronised and yet maintain their local time information.

If you were to set your Debian GNU/Linux computer to use local time, without taking account of timezones, you would lose the benefit of automatic DST changes. We do not recommend this! However, it may be necessary to compromise by setting your hardware clock to local time (see Multiboot with operating systems not understanding timezone, Section 16.3). In this document, we assume that you have configured your computer to use UTC.

To change the computer to use UTC after installation, edit the file /etc/default/rcS, change the variable UTC to no. If you happened to install your system to use local time, just change the variable to yes to start using UTC. It is best to reboot after editing /etc/default/rcS to get the changes effective.

If the Timezone is correctly set up, and the timezone configuration files are reasonably current, the local time shown by the operating system will change to Daylight Saving Time and back to normal time automatically on the correct dates. If the timezone files you have are old, there may be problems because DST start and end dates are not determined by a physical phenomenon, but are chosen by national institutions. Sometimes these dates are changed, for example, the European Union changed the end date from the last Sunday in September to the last Sunday in October in 1995.

For this reason, you should make sure that your libc6 package is kept reasonably up to date. This package, from Debian GNU/Linux 2.2 onwards, contains the timezone data.

16.1.1 Changing the timezone after installation

If the timezone is not set or is wrong, the superuser can run tzconfig to configure it after the operating system is installed (see man page tzconfig(8)).

If there are other users, it is a good idea to notify then that the system Timezone has changed.

16.1.2 Setting the TZ environment variable.

If you do not have root privileges or want to set for yourself a different timezone than the one the system uses, you can set the environment variable TZ. Use the command tzselect to show what value to use for TZ.

Example on using TZ:

     $ date
     Sun Nov  1 19:49:38 EET 1998
     $ export TZ=PST
     $ date
     Sun Nov  1 17:49:59 PST 1998

As you can see, for this user date shows a different time. (but this seems wrong, since Pacific Standard Time should be 09:49???. What's happening???).

16.2 Setting and showing hardware clock

The command for setting the hardware clock is hwclock. (This was not in Debian before 2.0. In earlier releases, the command was clock.)

If you use the date command to change time, it is worth setting also the hardware clock to the correct time. Otherwise, the time is wrong after the next reboot, since the hardware clock keeps the time when power is turned off. When the clock in the operating system shows the correct time, set the hardware clock like this:

     dilbert# date
     Sun Nov  1 18:56:50 EET 1998
     dilbert# hwclock --systohc --utc

Remember to add the --utc -option if the hardware clock is set to UTC!

See man page hwclock(8) for more information.

At least from Debian version 2.2 onwards, the system automatically saves the system time to hardware clock on shutdown, and sets the system clock from hardware clock when Debian boots up. This is done in the script /etc/init.d/hwclock.sh.

16.3 Multiboot with operating systems not understanding timezone

If you have a multiboot configuration, where you have some other operating system(s) on the same computer, and during boot choose which one to start, you may get confused about the correct time. If the other operating systems do not understand timezones, they think UTC is the local time and show the wrong time (unless you live in London, England or some other place in the same timezone).

DOS, OS/2 and Windows except NT do not use timezones. If you need to have the correct local time in these operating systems, you have to set the clock to local time. Debian Linux can live with this and it works, but if your main operating system(s) understand timezones, use them.

16.4 Syncing time, rdate and NTP

If you have another Unix computer which you know keeps the correct time, with root privileges you can set the time with the command rdate. For example:

     rdate somehost.domain.com

Even if you do not have an accurate time source, it is still a good idea to set all your computers to the same time, so that you can compare time stamps between hosts. To keep the clocks syncronized, You can start rdate once daily from cron.

16.4.1 Setting time at system boot

If the hardware clock does not keep the correct time, it is possible to set the correct time when the operating system boots. For this, there must be some other host in the LAN where the time can be received. Here is an example:

  1. create the file /etc/init.d/rdate

         $ cat rdate
         #! /bin/sh
         # rdate         Execute the rdate command.
         # Version:      1998-11-01 tapio.lehtonen@iki.fi
         # Set operating system time from other host in LAN.
         if [ -x /usr/sbin/rdate ]
                 /usr/sbin/rdate $HOST
  1.      chmod a+rx /etc/init.d/rdate
  1. Create a symbolic link to that file in directory /etc/rc2.d

         cd /etc/rc2.d
         ln -s ../init.d/rdate S19rdate
  1. At next reboot, you should see the time being set.

Remember to set execute permission to /etc/init.d/rdate. If You are wondering what the number S19 above means, read chapter ???. The key is to run rdate after netbase which starts networking, and before xntp3 or some other process where time is needed starts.

16.4.2 Setting time using NTP

If you are connected to the Internet, you can install an NTP client, for example ntp or xntp3 in Debian version 2.1 and older. This uses the Network Time Protocol RFC 1305 to synchronise clocks to a few tens of milliseconds precision. If you need this kind of precision or better, see also Radio clocks, Section 16.6.

See the excellent documentation that comes with ntp, in the Debian package ntp-doc. It is very thorough and thus long. If you think reading documents is a waste of time, just ask your Internet Service Provider or system administrator for NTP server names, or look up the nearest one in "List of Public NTP Servers" in http://www.eecis.udel.edu/~mills/ntp/servers.html . If you start using an NTP server, it is usually polite to notify the server's administrator of the fact.

Do not configure your system to query level 1 NTP servers! If you think you need to do this, you are almost certainly wrong!

Once you know an NTP server, edit the file /etc/ntp.conf to add at least one server line. Here is an example:

     # /etc/ntp.conf, configuration for xntpd
     logfile /var/log/xntpd
     driftfile /var/lib/ntp/ntp.drift
     statsdir /var/log/ntpstats/
     statistics loopstats peerstats clockstats
     filegen loopstats file loopstats type day enable
     filegen peerstats file peerstats type day enable
     filegen clockstats file clockstats type day enable
     server ntp.somedomain.something
     server ntp.something.else

Note, that I changed the actual server name in the above example, to prevent all Debian GNU/Linux users from blindly using that one server. Everything else except the server lines was there after xntp3 installation.

If you do not have a permanent Internet connection, then running NTP client is not a good solution. NTP client syncronizes relatively often, and needs the Internet connection to be always on. If you have a dial up Internet connection, you can run the command ntpdate (also in ntp package) to syncronize each time you connect.

Another possibility is to set cron to run ntpdate once daily, this gives good accuracy for most uses. The following is an example script that can be started from cron and runs ntpdate. This scripts assumes the /etc/ntp.conf is correctly set up, since it gets the server names from that file.

     dilbert# cat /etc/cron.nightly/ntpdate
     # Last modification: Sat Aug  8 05:27:07 EEST 1998
     # ntpdate  cron nightly
     if [ -f $NTPCONFFILE ] ; then
       echo "===========================" >> $LOGFILE
       echo "<<< `date` " >> $LOGFILE
       for i in `grep ^server $NTPCONFFILE | $CUT --fields 2 --delimiter \ `
             NTPSERVERS="$NTPSERVERS $i"
       echo ">>> `date` " >> $LOGFILE
     exit 0

16.4.3 Other methods

Debian GNU/Linux has also the command netdate, see man page netdate(8L) for more info. It uses the protocol defined in RFC 868.

16.5 Setting up an NTP server

If you have several hosts that you want to synchronise, you should not let them all separately synchronise to an outside NTP server. This puts a load on the NTP server, and if it is a public and free service, the people there may start to dislike you! At least ask if it is OK to sync all of your machines there.

A better way is to set up one of your hosts as an NTP server, sync that to the outside NTP server and the rest of your hosts to your own NTP server. The ntp package provides the server component as well, so you just need to configure it. This is not particularly difficult either, but you should read the documentation and understand the peer concept and stratum hierarchy.

16.6 Radio clocks

If you want better accuracy or cannot synchronise with an NTP server in the public Internet, you can purchase a radio clock. These get the time from time servers that use a very accurate clock, and broadcast time signals using radio waves.

There are radio clocks that need a ground based transmitter near enough to get the signal. If you get one of these, make sure you can receive the signal on the location where you plan to use it. The transmitters are ground based, so they don't work accross the globe.

Another kind of radio clock uses a Global Positioning System GPS receiver. GPS satellites circle the globe in low earth orbits, and there should always be at least four above the horizon. They are mainly used for navigation, but the principle they work on is based on sending time signals. For time synchronisation it is enough to get the signal from one satellite, but usually it is necessary to install the GPS antenna outdoors to get good reception.

16.7 Timestamps

Timestamps in files are in UTC. The command ls -l shows file timestamps in local time using the Timezone in effect. Because timestamps are in UTC, timestamps can be compared between separate computers. This can lead to problems if the clocks are not syncronized.

Consider this situation: You log in host B, which NFS loads your home directory from NFS server host A. You edit some file, save it and start make. If the clocks in hosts A and B are not syncronized, make may complain that the file modification time is in the future, or not compile the file because the object is newer.

16.8 Time in cron

The times in cron are local times. This means, that if you configure cron to start some program at 7 in the morning, it is 7 in the morning local time as determined by the system Timezone.

This causes headaches, if the system Timezone is not the same you have set for yourself using the TZ enviroment variable, or you have two host in different Timezones that need to start someting at the same time on both hosts.

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Debian GNU/Linux System Administrator's Manual (Obsolete Documentation)

This manual is OBSOLETE and DEPRECATED since 2006, 29 Dezember 2009. Instead see http://www.de.debian.org/doc/user-manuals#quick-reference.

Ardo van Rangelrooij ardo.van.rangelrooij@tip.nl
Tapio Lehtonen Tapio.Lehtonen@IKI.FI
Oliver Elphick - Previous maintainer