Exact Age Difference: Days, Months, Leap Years and More

Choosing a unit

• Years only: the everyday answer — “three years apart”. Rounds away up to almost a year of information.
• Years and months: the natural precision for birthdays — “3 years, 7 months”.
• Years, months, days: exact — “3 years, 7 months, 12 days”.
• Total days: one unambiguous number, useful for comparison and math (our calculator reports it alongside the calendar breakdown).

Converting between units

There is no clean single conversion factor, because months have 28–31 days and years have 365 or 366. As approximations: 1 year ≈ 365.25 days; 1 month ≈ 30.44 days. For a precise days figure, count actual days between the two dates — leap years shift the total. Going the other way, “2,500 days apart” is best converted by counting calendar years and months rather than dividing by a fixed constant.

Leap years and February 29

Two edge cases recur. First, when the earlier date falls on February 29 of a leap year and you step forward through a non-leap year, the anniversary has no exact February 29 to land on. This site’s convention — used by our calculator and stated openly — is to treat February 29 anniversaries in non-leap years as falling on March 1. Second, when counting days, leap years add February 29 to the total automatically, which is why the day-count method never drifts.

Who is older, and birthday order

The gap itself is symmetric — A is as much older than B as B is younger than A — but phrasing depends on order, so our calculator always names who was born earlier. When two people share a birthday (same month and day, including the February 29 convention above), the calculator says so explicitly.

Ready to compute? The age difference calculator applies all of these rules automatically, and the hand-method guide shows the arithmetic behind them. If your two dates are not birthdays — say, a project start and an end — see age difference between two dates for the calendar-date interpretation.