UTC, Coordinated Universal Time, is used as the official world reference for time. The time in each time zone is based on its distance from the UTC time zone, and each time zone is labeled based on the difference in hours between it and the UTC time zone. For example, UTC-4 and UTC-9 are four hours and nine hours behind UTC respectively.Continue Reading
When global time zones were first delineated in the late 1800s, they were based around GMT, Greenwich Mean Time. This standard was due primarily to the Greenwich Observatory's reputation for reliability and accuracy and is also the reason that the Prime Meridian runs through Greenwich.
Greenwich Mean Time is based on the earth's rotation and celestial measurements and was replaced by the more accurate Coordinated Universal Time in 1972. Coordinated Universal Time, also referred to as Zulu time or Z-time, is based on cesium-beam atomic clocks with leap seconds added to match earth-motion time as closely as possible.
The official global time zones, as well as the Prime Meridian, were established at the International Meridian Conference in 1884, which created the 24-hour time zone system with all zones referring back to GMT. Each time zone covers 1/24 of the globe, roughly 15 meridians, but zones are sometimes shifted to account for national boundaries or geography. China, for instance, operates entirely on one time zone despite being larger than 15 degrees of longitude.Learn more about Time Zones