Date of birth, abbreviated as eight characters, refers to the calendar date when a person is born; Year, month, year and day have * * * four columns, each with two characters, which are called * * * eight characters. Birthday plays an important role in Han folk beliefs, and ancient Han astrologers calculate people's fate accordingly. The expression of the eight characters is four pillars of Japanese law, four pillars of lunar law, or four pillars of Japanese law and lunar law. In order to get the correct birth time, the ancient Han people thought that the sundial instrument was the most accurate measurement during the day. When the clock time is the artificial average time and the regional standard, it is necessary to calculate the "apparent solar time difference" according to the solar terms (ecliptic) and the "local longitude time difference" according to the place of birth, so as to get the real astronomical birth time.