What time zone?
GMT (Greenwich Mean Time), mean solar time at the meridian from which time in other zones is calculated.
The times of various events, particularly astronomical and weather phenomena, are often given in "Universal Time" (abbreviated UT) which is sometimes referred to, now colloquially, as "Greenwich Mean Time" (abbreviated GMT).
GMT was established to aid worldwide oceanic navigation and is based on the rotation of the Earth. It was not until the British railway system started using GMT in the mid 1800s that GMT was adopted "for land use". And it was not until the late 1800s that GMT was adopted universally. At this time, the 24 time zones were created along with the International Date Line.
prime meridian
Greenwich mean time or Greenwich meridian time (GMT), the former name for mean solar time at the original site of the Royal Observatory in Greenwich, England, which is located on the prime meridian. In 1925 the numbering system was changed to make GMT equivalent to civil time at the prime meridian, and in 1928 the International Astronomical Union changed the designation of the standard time of the prime meridian to universal time (UT), which is now in general use.
Universal Time -- from Eric Weisstein's World of Astronomy
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