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<!DOCTYPE HTML PUBLIC "-//Norman Walsh//DTD DocBook HTML 1.0//EN">
<HTML
><HEAD
><TITLE
>Showing and setting time</TITLE
><META
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>The Linux System Administrators' Guide</TH
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><A
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>Chapter 11. Keeping Time</TD
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><HR
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><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="AEN2457"
>Showing and setting time</A
></H1
><P
> In the Debian system, the system time zone is determined
by the symbolic link <TT
CLASS="FILENAME"
>/etc/localtime</TT
>.
This link points at a time zone data file that describes
the local time zone. The time zone data files are stored in
<TT
CLASS="FILENAME"
>/usr/lib/zoneinfo</TT
>. Other Linux distributions
may do this differently. </P
><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
<TT
CLASS="FUNCTION"
>tzset</TT
> manual page. </P
><P
> The <B
CLASS="COMMAND"
>date</B
> command shows the current date and
time.
<A
NAME="AEN2466"
HREF="#FTN.AEN2466"
>[1]</A
>
For example:
<PRE
CLASS="SCREEN"
><TT
CLASS="PROMPT"
>$</TT
> <TT
CLASS="USERINPUT"
><B
>date</B
></TT
>
<TT
CLASS="COMPUTEROUTPUT"
>Sun Jul 14 21:53:41 EET DST 1996</TT
>
<TT
CLASS="PROMPT"
>$</TT
></PRE
>
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).
<B
CLASS="COMMAND"
>date</B
> can also show the univeral time:
<PRE
CLASS="SCREEN"
><TT
CLASS="PROMPT"
>$</TT
> <TT
CLASS="USERINPUT"
><B
>date -u</B
></TT
>
Sun Jul 14 18:53:42 UTC 1996
<TT
CLASS="COMPUTEROUTPUT"
>Sun Jul 14 18:53:42 UTC 1996</TT
>
<TT
CLASS="PROMPT"
>$</TT
></PRE
>
<B
CLASS="COMMAND"
>date</B
> is also used to set the kernel's software
clock:
<PRE
CLASS="SCREEN"
><TT
CLASS="PROMPT"
>#</TT
> <TT
CLASS="USERINPUT"
><B
>date 07142157</B
></TT
>
<TT
CLASS="COMPUTEROUTPUT"
>Sun Jul 14 21:57:00 EET DST 1996</TT
>
<TT
CLASS="PROMPT"
>#</TT
> <TT
CLASS="USERINPUT"
><B
>date</B
></TT
>
<TT
CLASS="COMPUTEROUTPUT"
>Sun Jul 14 21:57:02 EET DST 1996</TT
>
<TT
CLASS="PROMPT"
>#</TT
></PRE
>
See the <B
CLASS="COMMAND"
>date</B
> 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. </P
><P
> <B
CLASS="COMMAND"
>date</B
> only shows or sets the software
clock. The <B
CLASS="COMMAND"
>clock</B
> 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 <B
CLASS="COMMAND"
>date</B
>, and then the hardware clock with
<B
CLASS="COMMAND"
>clock -w</B
>. </P
><P
> The <SPAN
CLASS="OPTION"
>-u</SPAN
> option to <B
CLASS="COMMAND"
>clock</B
>
tells it that the hardware clock is in universal time.
You <I
CLASS="EMPHASIS"
>must</I
> use the <SPAN
CLASS="OPTION"
>-u</SPAN
>
option correctly. If you don't, your computer will be quite
confused about what the time is. </P
><P
> The clocks should be changed with care. Many parts of a
Unix system require the clocks to work correctly. For example,
the <B
CLASS="COMMAND"
>cron</B
> 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 <B
CLASS="COMMAND"
>cron</B
> wanted to run all the periodic
commands for twenty years all at once. Current versions of
<B
CLASS="COMMAND"
>cron</B
> can handle this correctly, but you should
still be careful. Big jumps or backward jumps are more dangeours
than smaller or forward ones. </P
></DIV
><H3
>Notes</H3
><TABLE
BORDER="0"
CLASS="FOOTNOTES"
WIDTH="100%"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN2466"
HREF="x2457.html#AEN2466"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>Beware of the <B
CLASS="COMMAND"
>time</B
> command, which does
not show the current time.</P
></TD
></TR
></TABLE
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