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World Time Zone Map

A time zone is a region on Earth that has a uniform, legally mandated standard time.Almost all time zones on land have legally defined borders which coincide with the borders of the country mandating the time or some subdivision thereof. Of the 40 time zones on land, most are offset from Coordinated Universal Time (UTC) by a whole number of hours (UTC-12 to UTC+14), but a few are offset by 30 or 45 minutes from a nearby hourly zone. In addition to land time zones, there are 25 nautical time zones, all separated by lines of longitude. Most (UTC-11 to UTC+11) are 15o of longitude wide, which is one hour of Earth's rotation relative to the Sun, but an hourly zone in the central Pacific Ocean is split into two 7.5o wide zones (UTC+12/-12) by the 180th meridian, part of which coincides with the International Date Line. Many land time zones are skewed toward the west relative to the corresponding nautical time zones.

Before 1972, all time zones were specified as an offset from Greenwich Mean Time (GMT), which was the mean solar time at the meridian passing through the Royal Observatory in Greenwich, London, United Kingdom. Since 1972, all official time services have broadcast radio time signals synchronized to UTC, a form of atomic time that includes leap seconds to keep it within 0.9 seconds of this former GMT, now called UT1. Many countries now legally define their standard time relative to UTC, although some still legally refer to GMT, including the United Kingdom itself. UTC, also called Zulu time, is used everywhere on Earth by astronomers and others who need to state the time of an event unambiguously.

The World Time Zone Map

Originally, all time on Earth was some local apparent solar time, the time on a sundial, so every city had its own time. When well-regulated mechanical clocks became widespread in the early 19th century, each city began to use some local mean solar time. The first time zone was created in 1847 by railroads on the island of Great Britain using GMT. Sandford Fleming of Canada proposed worldwide hourly time zones in 1879. By about 1900, almost all time on Earth was in the form of standard time zones, only some of which used an hourly offset from GMT. Many applied the time at a local astronomical observatory to an entire country, without any reference to GMT. It took many decades before all time on Earth was in the form of time zones referred to some "standard offset" from GMT/UTC. Nepal was the last country to adopt a standard offset, shifting slightly to UTC+5:45 in 1986. Daylight saving time or Summer Time, an advance of one hour, is mandated by many countries between spring and autumn. Computer operating systems use either UTC or a local time zone to time stamp events.