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<H1><A NAME="SECTION001220000000000000000">The hardware and software clocks</A></H1>
<P>
A personal computer has a battery driven hardware clock. The
battery ensures that the clock will work even if the rest of
the computer is without electricity. The hardware clock can
be set from the BIOS setup screen or from whatever operating
system is running.
<P>
The Linux kernel keeps track of time independently from the
hardware clock. During the
boot, Linux sets its own clock to the same time as the hardware
clock. After this, both clocks run independently. Linux
maintains its own clock because looking at the hardware is
slow and complicated.
<P>
The kernel clock always shows universal time. This way, the
kernel does not need to know about time zones at all--the simplicity
results in higher reliability and makes it easier to update the
time zone information. Each process handles time zone conversions
itself (using standard tools that are part of the time zone
package).
<P>
The hardware clock can be in local time or in universal time.
It is usually better to have it in universal time, because
then you don't need to change the hardware clock when daylight
savings time begins or ends (UTC does not have DST). Unfortunately,
some PC operating systems--including MS-DOS, Windows, OS/2--assume
the hardware clock shows local time. Linux can handle either, but
if the hardware clock shows local time, then it must be modified
when daylight savings time begins or ends (otherwise it wouldn't
show local time).
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<P><ADDRESS>
<I>Lars Wirzenius <BR>
Sat Nov 15 02:32:11 EET 1997</I>
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