Question: When does the physical birthday actually begin and end? How many hours of the day does it cover?

Sri Chinmoy: Actually a birthday covers all the twenty-four hours. Suppose you were born at three o’clock in the afternoon. You do not have to take the twenty-four hours from that hour. Right after midnight you can begin to observe your birthday. When the calendar falls, after twelve o’clock, that is your birthday.

From:Sri Chinmoy,The soul's special promise, part 2, Agni Press, 1999
Sourced from https://srichinmoylibrary.com/ssp_2