GMT Full Form is Greenwich Meridian Time. GMT refers to the mean solar time in Greenwich, London. Greenwich in England has been the GMT home since 1884. It is also referred as Greenwich Mean Time. It is measured from Greenwich Meridian Line in the Royal Observatory located in Greenwich.
To simply put, it is the base place, from which the time zones around the world are measured. The prime meridian here is the Longitude Zero Degrees. GMT is the average time taken by the earth to rotate on its axis in 24 hours. Greenwich, England is located in: (1) Longitude 00 0’0” Latitude 510 28’ 38”N (North of Equator)