Mean solar time at the village of Greenwich near London which, as decided at the International Meridian Congress of 1884, is assigned 0 degrees longitude (called Prime Meridian). Because it is based on the Earth's rotation (which is irregular) it actually follows a fictitious Mean Sun that moves at a uniform speed along the equator. A 19th century worldwide time standard, it was replaced in 1970 by the Coordinated Universal Time (UTC) for astronomical and navigational use, but is still in common use by airlines and radio and television stations. When expressed in terms of a 24-hour clock, GMT is also called universal time (UT or Z). It is five hours ahead of the US Eastern Standard Time (EST) during standard period, and four hours ahead during daylight-saving period.
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