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Local Mean Time

Local mean time is a form of solar time that corrects the variations of local apparent time. Local mean time was used from the early nineteenth century, when local solar time or sundial time was last used, until standard time was adopted on various dates in several countries.

Standard Time

Standard time means that the same time is used throughout the same region. Prior to the introduction of standard time, every municipality set its clock, if it had one, by the local position of the sun (Local Mean Time). This served well until the introduction of the train, when it became possible to travel fast enough to require almost constant re-setting of clocks. Standard time, where all clocks in a region used the same time, was invented to solve this problem.


To convert to Eastern Standard Time (EST), add 10 hours to UTC.
To convert to Central Standard Time (CST), add 91/2 hours to UTC.
To convert to western Standard Time (WST), add 8 hours to UTC.

A scheme was devised where the surface of the planet was divided into twenty four "time zones", each separated by 15° of longitude and offset by one hour from its neighbour. Under this scheme, local time is always close to mean solar time, while comparing the time in different places is a simple matter of adding or subtracting whole hours. However, the one hour separation is not universal and, as the map below shows, the shapes of time zones can be quite irregular because they usually follow the boundaries of states, countries or other administrative areas.

Local Summer Time

Daylight saving time (also called DST) is the Australia term for a system intended to "save" daylight. The official standard time is adjusted forward one hour, remaining that way for the duration of the spring and summer months. This is intended to provide a better match between the hours of daylight and the active hours of work and school. DST is most commonly used in temperate regions, due to the considerable variation in the amount of daylight versus darkness through the seasons in those regions.

Those states marked with a dark shade on the map use Australian Summer Time (also known as Daylight Saving Time) to advance their clocks one hour on Sunday 2 October and when DST finishes put them back an hour on Sunday 2 April.

UTC

Universal Time Coordinated or UTC, also sometimes referred to as "Zulu time" or Z, is an atomic realization of Greenwich Mean Time (GMT), the astronomical basis for civil time. Time zones around the world are expressed as positive and negative offsets from GMT. UTC differs by an integral number of seconds from International Atomic Time (TAI), as measured by atomic clocks and a fractional number of seconds from GMT.

Greenwich Mean Time (GMT) is mean solar time at the Royal Greenwich Observatory in Greenwich, London, England, which by convention is at 0 degrees geographic longitude. Noon Greenwich Time was thought of as the moment when the Sun crosses the Greenwich meridian and reaches its highest point in the sky; however because of Earth's uneven speed in its elliptic orbit and its axial tilt may cause this event to be up to 16 minutes away from noon. The fictitious mean sun is now the annual averages of this non-uniform motion of the true Sun, necessitating the inclusion of the word “mean” (average) in Greenwich Mean Time.

The Earth's rotation is very slowly decelerating (due to braking action of the tides); hence the mean solar day has increased since TAI was introduced. For this reason, GMT is 'slower' than TAI. UTC is maintained within 0.9 s of GMT; leap seconds are added (or, theoretically, subtracted) at the end of any UTC month as necessary. However a leap second has not been required since 1998, as the deceleration of the Earth's rotation slowed temporarily in the past seven years. The International Earth Rotation and Reference Systems Service announced in July 2005 that the next leap second will be on 31 December 2005.

For most practical and legal-trade purposes, the fractional difference between UTC and GMT is inconsequentially small, and for this reason UTC is colloquially called GMT sometimes, even if this is not technically correct.