How age is calculated

Count completed years first

A person turns one year older only when the anniversary of the birth date is reached. Before that anniversary, subtract one from the simple year difference.

Then borrow real calendar months

If the as-of day is earlier in the month than the birth day, borrow the actual number of days in the preceding month. That is why month length changes the days remainder.

Leap years affect elapsed days

Leap days change the total-day interval but do not make a birthday arrive early. Policies for a February 29 birthday can depend on jurisdiction, so the calculator reports neutral calendar math.

Useful distinction: “25 years old” and “9,131 days elapsed” answer different questions.