What is a tithi birthday?
Many Hindu families celebrate birthdays by the lunar calendar: on the day the tithi you were born on comes round again in the same lunar month. Because the lunar year is about eleven days shorter than the solar year, and an extra month is added every few years, the Gregorian date moves each year, usually within a range of about a month.
How it is worked out
The page finds the tithi and the amanta lunar month at your moment of birth, then, for each year, the day in that month when the same tithi is running at sunrise in the city where you celebrate. If the tithi is skipped that month (a kshaya tithi), the day it begins is used. If you were born in an Adhik maas, the regular month of the same name is used, as most families do.
Nakshatra birthday
In Tamil Nadu, Kerala and much of South India the birthday follows the Janma nakshatra instead: the day your birth nakshatra is running at sunrise while the Sun is in the same sign as at your birth. It is shown alongside for the next three years.