Time Zones are regions in the Earth that use the same local time.
Conventionally, people compute their local time as an offset from UTC
(Coordinated Universal Time). UTC replaced GMT (Greenwich Mean Time), primarily
because UTC is considered an actual time zone. UTC is a high precision
atomic time standard, which measures time by the nanosecond. UTC has standard
seconds as defined by the International Atomic Time, and has leap seconds (time
particle that measures time not on the passage of seconds, but on the Earth's
angular rotation) that are announced at irregular intervals for the compensation
of geographical occurrences, such as the earth's slowing rotation.
GMT, on the other hand, is the mean solar time at the 0 degree longitude
(known as the Prime Meridian). Solar-based time has been used all throughout the
history of time determination, the most known of which is the sundial. However,
the flaw lies in how the mean solar time is determined by the rotation of the
Earth. The Earth's rotation is not constant, and time is ideally, a constant
thing, wherever a person is in the earth. Knowing this, atomic clocks were
charged annually to closely match GMT. However, in 1972, a new system was
discovered, and this system involved the use of leap seconds, as mentioned
above.
As a general rule, adjacent time zones are exactly one hour apart. Standard
time zones are geometrically defined with the subdivision of the Earth's sphere
into 24 lunes (wedge shaped sections), surrounded by meridians that are 15
degrees of longitude apart from each other, hence the general rule of one-hour
time differences between neighbouring regions.
There are exceptions, however, for the benefit of those with geographical and
political issues. Some time zones are irregularly-shaped, due to establishment
of political boundaries, and seasonal time changes (DST or Daylight Saving
Time).
Time Zones are typically defined with two variations. The first meaning of
time zone is a uniform representation of a particular region, where the time is
determined offset from a global reference (usually UTC). On the other hand, one
that more people are familiar with, a time zone is a representation of a region
with a common, ever consistent local time, despite seasonal fluctuations of the
offset.
There are a total of 84 time zones in the world, including time zones with
DST. Provided that DST is not in effect, most time zones may be written in short
form as UTC +/-n (or GMT+/-n), such as Melbourne Australia, UTC+10, 22:00, and
Los Angeles, California, U.S.A. UTC-8, 04:00.
