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<H1><A NAME="SECTION001230000000000000000">Showing and setting time</A></H1>
<P>
In the Debian system, the system time zone is determined by the
symbolic link <tt>/etc/localtime</tt><A NAME="3159"> </A>. This link points at a time
zone data file that describes the local time zone. The time
zone data files are stored in <tt>/usr/lib/zoneinfo</tt><A NAME="3161"> </A>.
Other Linux distributions may do this differently.
<P>
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 <em>tzset</em><A NAME="3163"> </A>(3) manual page.
<P>
The <tt>date</tt><A NAME="3165"> </A> command shows the current date and
time.<A NAME="tex2html54" HREF="footnode.html#3157"><IMG ALIGN=BOTTOM ALT="gif" SRC="./foot_motif.gif"></A> For example:
<BLOCKQUOTE> <TT>
<code>$</code> <i>date</i> <BR>
<code>Sun Jul 14 21:53:41 EET DST 1996</code> <BR>
<code>$</code>
</TT></BLOCKQUOTE>
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).
<tt>date</tt><A NAME="3169"> </A> can also show the univeral time:
<BLOCKQUOTE> <TT>
<code>$</code> <i>date -u</i> <BR>
<code>Sun Jul 14 18:53:42 UTC 1996</code> <BR>
<code>$</code>
</TT></BLOCKQUOTE>
<tt>date</tt><A NAME="3171"> </A> is also used to set the kernel's software clock:
<BLOCKQUOTE> <TT>
<code>#</code> <i>date 07142157</i> <BR>
<code>Sun Jul 14 21:57:00 EET DST 1996</code> <BR>
<code>#</code> <i>date</i> <BR>
<code>Sun Jul 14 21:57:02 EET DST 1996</code> <BR>
<code>#</code>
</TT></BLOCKQUOTE>
See the <tt>date</tt><A NAME="3173"> </A> manual page for more details--the syntax
is a bit arcane. Only <tt>root</tt> can set the time. While
each user can have his own time zone, the clock is the same for
everyone.
<P>
<tt>date</tt><A NAME="3175"> </A> only shows or sets the software clock.
The <tt>clock</tt><A NAME="3177"> </A> commands syncronizes 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 <tt>date</tt><A NAME="3179"> </A>, and then the
hardware clock with <tt>clock -w</tt>.
<P>
The <tt>-u</tt> option to <tt>clock</tt><A NAME="3181"> </A> tells it that the
hardware clock is in universal time.
You <em>must</em> use the <tt>-u</tt> option correctly. If you
don't, your computer will be quite confused about what the
time is.
<P>
The clocks should be changed with care. Many parts of
a Unix system require the clocks to work correctly. For example,
the <tt>cron</tt><A NAME="3183"> </A> 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 <tt>cron</tt><A NAME="3185"> </A> wanted to
run all the periodic commands for twenty years all at once.
Current versions of <tt>cron</tt><A NAME="3187"> </A> can handle this correctly, but
you should still be careful. Big jumps or backward jumps are
more dangeours than smaller or forward ones.
<P>
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<P><ADDRESS>
<I>Lars Wirzenius <BR>
Sat Nov 15 02:32:11 EET 1997</I>
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