The world is divided into basically 24 time zones. For easy reference in communications, a letter of the alphabet has been assigned to each time zone. The "clock" at Greenwich, England is used as the standard clock for international reference of time in communications, military, aviation, maritime and other activities that cross time zones. The letter designator for this clock is Z.
Times written in military time (24 hour format) are four digits, such as, 1830Z (6:30 pm) with the Zulu suffix. Note that the phonetic alphabet is used for the letter Z (Zulu). This time is usually referred to as Zulu Time because of the letter assigned to this time zone. Its official name is Coordinated Universal Time or UTC. This time zone had previously been called Greenwich Mean Time (GMT) but was replaced with UTC in 1972 as the official world time standard changed. While GMT is based on Earth's rotation and celestial measurements, UTC is a based on cesium-beam atomic clocks. The two clocks are rarely more than a second apart as leap seconds are applied to UTC.