Birthday is also called eight characters, that is, eight characters corresponding to middle age, month, day and time in ancient heavenly stems and earthly branches are taken to record a person's date of birth. The eight characters are actually another way of saying "four pillars" in the terminology of Zhouyi. The four pillars are the year, month, day and time when a person was born. In heavenly stems and earthly branches, years, months, days and hours all have corresponding "dry branches", such as Jiazi Year, Shen Bingyue, Xin Ugly Month and Renyin Time. There are two words in each column and eight characters in four columns, so fortune telling is also called "measuring eight characters".
The theory of "birthday" began in the Tang Dynasty. At that time, there was an official named Xu Li in the court. He believes that a person's "fate" is determined by three factors: the date of his birth.
In the Song Dynasty, Xu Ziping developed this idea again. Among the three factors of "year, month and day", "time" is added, and each factor is represented by two words, such as "B ugly" for the year, "Jia Yin" for the month, "Xin ugly" for the day, and "Ding You" for the time, which becomes the so-called "birthday".