Age Difference Calculator

This gives the gap between two birthdates as years, months and days. It differs from an age calculator in that neither end is today: both are fixed dates, and the answer never changes. The question comes up more often than it looks. Parents work out the spacing between siblings, couples work out the gap between them, and it matters practically for anything where two people's entitlements are staggered, such as when each becomes eligible for New Zealand Superannuation at 65, or when school year cut-offs fall between two children. The result is given the way people say it, in years and months rather than as a total, because a gap of 1,271 days means nothing to anyone while three years and five months is immediately legible. Alongside that, the calculator shows how old the elder person was on the day the younger was born, which is usually the more interesting framing when the question is about siblings. Nothing here depends on today's date, so the answer you get is stable and can be quoted without a caveat about when it was calculated.

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3 years, 5 months, 13 days
age difference
Total days1,262
Total weeks180
Total months41
Elder was this old at the birth3 years, 5 months, 13 days
In decimal years3.46

Both dates are fixed, so the result never changes with the calendar. Counted whole years first, then whole months, then the days left over.

How it works

The earlier of the two dates is treated as the elder person. Whole years are counted from that date, reduced by one if the later date has not yet reached the same month and day. Whole months are then counted from that anniversary, again reduced if the day of the month has not been reached. What remains is the days, taken from the length of the actual preceding month rather than an average. Total days is the raw difference, and decimal years divides that by 365.25 to account for leap years across the period.

Worked example

Take the defaults, 20 May 1990 and 2 November 1993. Counting whole years from 20 May 1990 gives 3, reaching 20 May 1993. Counting whole months from there gives 5, reaching 20 October 1993. From 20 October to 2 November is 13 days. So the gap is 3 years, 5 months and 13 days. The elder was exactly that old on the day the younger was born, which is the same figure viewed from the other side.

Related calculators

Data sources: leap years follow the Gregorian rule. Decimal years use an average year length of 365.25 days.

Frequently asked questions

How do I work out the age gap between two people?

Count whole years from the earlier birthdate, then whole months from that anniversary, then the days left over. Do not divide the total days by 365, because leap years and varying month lengths make the result drift by several days over a decade.

Does this use today's date?

No. Both ends are fixed birthdates, so the answer is stable and never changes. That is the difference between this and an age calculator, which measures from a birthdate to today and therefore changes daily.

Why is the decimal figure divided by 365.25?

Because a year averages 365.25 days once leap years are included. Dividing by 365 would overstate the gap slightly, and the error grows the longer the period.