Glossary - Pierre DeRoche PDR 4020 Mode D'emploi

Glossary

GMT
The abbreviation "GMT" stands for Greenwich Mean Time and
refers to mean solar time at the Greenwich meridian crossing the
royal observatory near London and which constitutes the basis
for calculating longitudes. "Greenwich Mean Time" served as a
temporal reference until 1972, the year when it was replaced by
Universal Time Coordinated (UTC). Despite this new reference,
the term GMT is still widely used.
Historically speaking, the surface of the globe had been divided
into 24 equal-sized time zones, in which the hour was identical
throughout each. The dateline had initially been fixed at the
180° meridian. This subdivision was subsequently modified
to take account of the needs of certain countries wishing for
example to have only one legal time, despite the fact that their
territory extends over several time zones (such as China) or keen
that their legal time should be different from that theoretically
allocated (e.g. Spain). Moreover, some countries (such as Iran)
have adopted a legal time corresponding to 30 and 45-minute
fractions of a time zone.
There are currently 40 time zones, taking account of some that
exist only temporarily such as "summer time" (Daylight Saving
Time)/or winter time.
The choice of the dateline corresponds approximately to the 180°
meridian, located in the Pacific Ocean and distinguished by the
fact that it touches virtually no land. The line has nonetheless
been slightly diverted to avoid two islands.
13