Why age is counted this way
Age is measured in completed years, then completed months within the part-year, then the remaining days. Someone born on 15 June 1990 turns 35 on 15 June 2025 and stays 35 until the following June, regardless of how the months divide up. That is the convention used almost everywhere for legal age, and it is what this calculator applies.
Uneven months
Months have 28 to 31 days, so the day count borrows from the previous month when needed. From 31 January to 1 March is one month and one day in a non-leap year, because February contributed 28 days. Different tools handle this borrowing slightly differently, which is why two age calculators can disagree by a day on awkward dates.
Leap year birthdays
People born on 29 February have a birthday only every four years. Most jurisdictions treat 1 March as the legal birthday in common years, though a few use 28 February. This calculator counts forward to the next real 29 February for the birthday countdown.
Other uses for the same maths
Set the comparison date to a future date to find how old someone will be at a wedding, a school entry cut-off or a retirement date. Set it to a past date to find how old someone was when a particular event happened. To count the days or working days between two dates, use the date calculator instead.