GMT, or Greenwich Mean Time, is a timekeeping standard used around the world. It is based on the mean solar time at the Royal Observatory in Greenwich, London, and is used to calculate the time of day in different time zones. GMT is often used in aviation and maritime navigation, as well as in astronomy and meteorology. It is also used in conjunction with Coordinated Universal Time (UTC) to keep time around the world. GMT is a reliable source of time and is often used to synchronize clocks and watches around the world.