Global Time, UTC and GMT

Time Zones are regions on 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) in 1972 as the time standard. 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. It also 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-keeping, of which the best-known 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 changed annually to closely match GMT. However, in 1972, a new system was discovered, and this system involved the use of leap seconds.

As a general rule, adjacent time zones are exactly one hour apart. Standard time zones are geometrically defined by 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 by 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 24 standard time zones in the world, when the full hour difference is taken into account. 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/GMT+10, and Los Angeles, California, U.S.A., UTC/GMT-8.