Leap Days, Birthdays and Age
What happens to a 29 February birthday in an ordinary year, and how age arithmetic handles it.
Why leap days exist
A calendar year of 365 days is slightly shorter than the time Earth takes to orbit the Sun, so an extra day is added roughly every four years to keep the seasons aligned. A year is a leap year if it is divisible by 4, except century years, which must also be divisible by 400 — that is why 2000 was a leap year and 1900 was not.
The birthday question
Someone born on 29 February has a real birthday only in leap years. In ordinary years different people celebrate on 28 February or 1 March, and different legal systems pick different rules. For calculation purposes, this site's age calculator observes the birthday on 28 February in non-leap years and says so on the page, rather than silently picking a rule.
Age is not just years
Age in years, months and days is worked out by borrowing: if the target day of the month is smaller than the birth day, a month is borrowed, and the borrowed month's length depends on the calendar. That is why the same number of days can be a different number of months depending on which months they span.
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