Why do some people get married and everyone in the family have to help both parties count their birthdays? Date of birth and how to calculate it?

Birthdays are harmonious, and couples communicate well and care for each other. With harmony and happiness, the relationship between husband and wife is good or bad, and there is no harmony. Husband and wife have a short fate, divorce is a bit big, or husband and wife live in different places. When they are born, they can calculate the date of birth. You can also add my QQ number and ask more questions.