How many days until my birthday?

At midnight at the start of your birthday, on your own clock: the count runs in your device's time zone, so it ends the moment the day begins where you are. On the day itself it wishes you a happy birthday and starts counting to the next one.

A birthday moves one weekday later every year, because 365 days is 52 weeks and one day, and two weekdays later when a February 29 falls in between. A birthday on a Saturday this year is on a Sunday next year, unless a leap day comes first.

Only in leap years: every year divisible by four, except the century years not divisible by 400, so 2100 will not have one. In the years between, most people born on February 29 celebrate on February 28 or March 1. By our estimate, about 219,000 Americans alive today were born on one.