14.1. The concept of localtime
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.
Noon is the time of the day when the Sun is at its
highest position. Since (according to recent research) the Earth is
round, noon happens at different times in different places. This
leads to the concept of local time. 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.
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 universal
time (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.
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 daylight savings time,
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.
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.
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.