Age Difference Calculator

The age difference between two people is the older one's age on the day the younger was born. That is the whole method, and it is why the answer comes out in years, months and days rather than as a subtraction of birth years. Born September 14, 1988 and February 20, 1993: on February 20, 1993 the older person was 4 years, 5 months and 6 days old, so that is the gap, and it never changes.

Subtracting the birth years gives 1993 - 1988 = 5, which is wrong by most of a year. The years only tell you which calendar each person was born in. Counting forward from the earlier date instead, by whole years to September 14, 1992, then whole months to February 14, 1993, then the 6 days left, gives the exact figure. Counted in plain days it is 1,620, which takes in one leap day, February 29, 1992.

Ask either person their age and the difference will not always be 4. Between September 14 and February 20 each year, the older one has already had a birthday and the younger has not, so their ages differ by 5. From February 20 to September 13 they differ by 4. Two people whose ages differ by 5 at a dinner in December can be 4 years apart in June without either age being wrong. Only a pair born on the same day of the year has the same difference all year.

An age gap calculator takes the two dates in either order and gives the gap in whole months (53 here, plus 6 days) and in days and weeks (231 weeks and 3 days).

The same method works for any two start dates, such as siblings, or two companies by founding date.

Two birth dates on February 29 behave like any other date in the count, with one wrinkle: in a common year the leap-day birthday falls on February 28 in this calculator, so the window when the two ages differ by the larger number can shift by a day.