Knowing the total number of days, how to calculate when this person's birthday is?

How many days from birth to a certain day can be counted. If there is no cross-century these days, it is better to calculate.

The total number of days MINUS the number of days that passed in that year is the number of days until the end of last year, divided by 365.25. Integer is the year of birth, which can be determined, and the remainder can be calculated by month and day.