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<H1><A NAME="SECTION001210000000000000000">Time zones</A></H1>
<P>
Time measurement is based on mostly regular natural
phenomena, such as alternating light and dark periods
caused by the rotation of the planet. The total time
taken by two successive periods is constant, but the
lengths of the light and dark period vary. One simple
constant is noon.
<P>
Noon is the time of the day when the Sun is at its highest
position. Since the Earth is round,<A NAME="tex2html53" HREF="footnode.html#3113"><IMG ALIGN=BOTTOM ALT="gif" SRC="./foot_motif.gif"></A> noon happens at different times in
different places. This leads to the concept of <b>local
time</b>. Humans measure time in many units, most of which are
tied to natural phenomena like noon. As long as you stay in
the same place, it doesn't matter that local times differ.
<P>
As soon as you need to communicate with distant places, you'll
notice the need for a common time. In modern times, most of
the places in the world communicate with most other places in
the world, so a global standard for measuring time has been defined.
This time is called <b>universal time</b> (UT or UTC, formerly
known as Greenwich Mean Time or GMT, since it used to be local
time in Greenwich, England). When people with different local
times need to communicate, they can express times in universal
time, so that there is no confusion about when things should happen.
<P>
Each local time is called a time zone. While geography would
allow all places that have noon at the same time
have the same time zone,
politics makes it difficult. For various reasons, many countries
use <b>daylight savings time</b>, that is, they move their
clocks to have more natural light while they work, and then move
the clocks back during winter. Other
countries do not do this. Those that do, do not agree
when the clocks should be moved, and they change the rules
from year to year. This makes time zone conversions definitely
non-trivial.
<P>
Time zones are best named by the location or by telling the
difference between local and universal time. In the US and
some other countries, the
local time zones have a name and a three letter abbreviation.
The abbreviations are not unique, however, and should not be
used unless the country is also named. It is better to talk
about the local time in, say, Helsinki, than about East
European time, since not all countries in Eastern Europe follow
the same rules.
<P>
Linux has a time zone package that knows about all
existing time zones, and that can easily be updated when the
rules change. All the system administrator needs to do is to
select the appropriate time zone. Also, each user can set
his own time zone--this is important since many people work
with computers in different countries over the Internet.
When the rules for daylight savings time change in your local
time zone, make sure you'll upgrade at least that part of your
Linux system. Other than setting the system time zone and
upgrading the time zone data files, there is little need to
bother about time.
<P>
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<P><ADDRESS>
<I>Lars Wirzenius <BR>
Sat Nov 15 02:32:11 EET 1997</I>
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