Greenwich, London, England, is located in the southeast of London, on the south bank of the Thames. The Royal Greenwich Observatory was established in 1675- 1948. 1884 The International Longitude Conference held in Washington decided to take the meridian passing through Greenwich as the prime meridian, which is also the starting point for calculating time and geographical longitude in the world.

Greenwich mean time (old translation Greenwich mean time or commonly known as Greenwich mean time; English: Greenwich Mean Time (GMT) refers to the standard time of the Royal Greenwich Observatory in the suburb of London, England, because the prime meridian is defined as the meridian passing through it. Starting from February 5th, 1924, Greenwich Observatory will release time adjustment information to the world every hour. Greenwich mean time is no longer used as standard time because the daily rotation of the earth is somewhat irregular and slowly slows down. The current standard time is Coordinated Universal Time (UTC), which is reported by the atomic clock.