Because different countries and regions in the world have different longitudes and local time, they will be divided into different time zones. There was no time zone in ancient times, and the formal time zone division included 24 time zones, each of which was represented by an English letter.

Every longitude 15 minutes has a time zone, with one exception, and each time zone has a central meridian; For example, GMT belongs to the "Z" area, so the suffix "Z" is usually added after its time (the suffix "Zulu" is used in spoken language).

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Since the Western Zhou Dynasty, China's calendar has gradually stabilized the concept of twelve months a year. Taking a day as a year, we also divide a day by twelve o'clock, and fix the date demarcation point (date change point) at midnight.

Twelve time nodes (starting points) at twelve o'clock are twelve o'clock. In the Han Dynasty, it was named Midnight, Crow, Pingdan, Sunrise, Food Time, Horn, Japan-China, Sundial, Sunset, Dusk and Man's Decision.

With the diversification of people's work and life, their words are gradually unsatisfactory, so they are represented by twelve earthly branches, including eleven o'clock in the evening, one o'clock in the morning, three o'clock in the morning, five o'clock in the morning, seven o'clock in the morning, nine o'clock in the morning, eleven o'clock in the afternoon, three o'clock in the afternoon, five o'clock in the evening, seven o'clock in the evening and nine o'clock in the evening.

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