On 29 Feb 2008, I asked Georgia how old she was. “I’m four, but will be five tomorrow!” Georgia was born on 1 March 2003. She doesn’t celebrate her 5th birthday until 1 March 2008, despite the fact that an extra day sneaks in just before then. On 29 Feb 2008, she’s 4 years and 365 days old. On 1 March 2008, she’s 5 years and 0 days old.

Anant was born on 29 February 2000. On 28 February 2007, ask him how old he is, and he’ll say, “Six!”. Ask him on 1 March 2007, and he’ll say, “Seven! And where’s my birthday present?” But, when was his birthday? Actually, sorry, but it wasn’t. People born on Feb 29 really do only have a birthday, the same date as they were born, every four years. They jump from being 6 years and 364 days old on Feb 28 to being 7 years and 1 day old on March 1, without ever being 7 years and 0 days old, that year. Anant might have a birthday party, he might have nice presents, but he doesn’t have a birthday.

The birthdate calculator understands leap years and (I hope!) gives the right answer for all leap-year-related birthdays.