Because a person's fortune and personality are born to a certain extent and have been set at birth. Although many people will say "my fate is not determined by heaven", people who believe in geomantic numerology believe this very much.
Young people generally don't know when they were born, and they can infer from the number of brothers and sisters when their parents say it is day or night. However, this method also has a drawback, that is, family planning is implemented after the 1980s, and many people are only children without brothers and sisters.
Furthermore, it can be inferred from everyone's personality. For example, people born in childhood are rigid, impatient, often vacillating and have no opinions. Parents and wives are very helpful in life.
People born in Yin Shi have a normal relationship with their families, and many people have left their homes. You may have a mediocre career when you are young, but you will gradually change in your later years. People born at the age of four are usually smart and have great potential in their careers. But because I was too busy, I alienated my relatives and friends.
But these can only be used as a basis and cannot be completely concluded. Because some people's personality will change, after a big event at home, the originally impatient personality will become very gentle.
So these can only be used as reference. If you really want to know your birthday, you can tell all the information you can provide to the person who specializes in calculation, and they will give a relatively accurate result.