The earth rotates from west to east, and the sun appears earlier in the east than in the west, and the time in the east is earlier than in the west. The difference between eastern time and western time is not only calculated in hours, but also in minutes and seconds, which brings inconvenience to people.

In order to overcome the time confusion, an international longitude conference (also known as the International Meridian Conference) held in Washington in 1884 stipulated that the world should be divided into 24 time zones (East 12 time zone and West 12 time zone). Britain (the former site of Greenwich Observatory) is defined as the central time zone (zero time zone), East 1- 12 and West 1- 12.

Each time zone spans longitude 15 degrees and the time is exactly 1 hour. The last east-west zone 12 spans longitude 7.5 degrees respectively, and is bounded by east-west longitude 180 degrees. The time on the central meridian of each time zone is the time uniformly adopted in this time zone, which is called time zone. The time difference between two adjacent time zones is 1 hour.