How you calculate your birthday each year?

It may seem a simple question. But it is not the case.
In Kerala, Hindus calculate their birthday every year, the same way we calculate Srikrishna Jayanthi or Mahasivarathri.
i.e. each year, our birthday differs and most probable times, it will never be on our date of birth.
We calculate our birthday on the basis of Malayalam calender and born star. What about you?

How do you calculate your birthday every year...Is it just your date of birth?