Showing and setting time

Linux System Administrators Guide:
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# 14.3. Showing and setting time

In Linux, the system time zone is determined by the symbolic link `/etc/localtime`. This link points to a time zone data file that describes the local time zone. The time zone data files are located at either `/usr/lib/zoneinfo` or `/usr/share/zoneinfo` depending on what distribution of Linux you use.

For example, on a SuSE system located in New Jersey the `/etc/localtime` link would point to `/usr/share/zoneinfo/US/Eastern`. On a Debian system the `/etc/localtime` link would point to `/usr/lib/zoneinfo/US/Eastern`.

If you fail to find the `zoneinfo` directory in either the `/usr/lib` or `/usr/share` directories, either do a find /usr -print | grep zoneinfo or consult your distribution's documentation.

What happens when you have a users located in a different timezone? A user can change his private time zone by setting the TZ environment variable. If it is unset, the system time zone is assumed. The syntax of the TZ variable is described in the `tzset` manual page.

The date command shows the current date and time. For example:

    $ date
    Sun Jul 14 21:53:41 EET DST 1996
    $

That time is Sunday, 14th of July, 1996, at about ten before ten at the evening, in the time zone called ``EET DST'' (which might be East European Daylight Savings Time). date can also show the universal time:

    $ date -u
    Sun Jul 14 18:53:42 UTC 1996
    $

date is also used to set the kernel's software clock:

    # date 07142157
    Sun Jul 14 21:57:00 EET DST 1996
    # date
    Sun Jul 14 21:57:02 EET DST 1996
    #

See the date manual page for more details; the syntax is a bit arcane. Only root can set the time. While each user can have his own time zone, the clock is the same for everyone.

Beware of the time command. This is not used to get the system time. Instead it's used to time how long something takes. Refer the the time man page.

date only shows or sets the software clock. The clock commands synchronizes the hardware and software clocks. It is used when the system boots, to read the hardware clock and set the software clock. If you need to set both clocks, you first set the software clock with date, and then the hardware clock with clock -w.

The `-u` option to clock tells it that the hardware clock is in universal time. You must use the `-u` option correctly. If you don't, your computer will be quite confused about what the time is.

The clocks should be changed with care. Many parts of a Unix system require the clocks to work correctly. For example, the cron daemon runs commands periodically. If you change the clock, it can be confused of whether it needs to run the commands or not. On one early Unix system, someone set the clock twenty years into the future, and cron wanted to run all the periodic commands for twenty years all at once. Current versions of cron can handle this correctly, but you should still be careful. Big jumps or backward jumps are more dangerous than smaller or forward ones.

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