Age Difference Calculator

Enter two dates of birth to see the exact age difference between them in years, months and days, along with the same gap as total days, weeks and completed months.

You can type the dates in either order. The calculator works out who is older and measures the age gap forward from the earlier date, using the real lengths of the calendar months.

Last updated October 19, 2026

Examples

Two siblings

Person 1 date of birth:: 2012-09-14
Person 2 date of birth:: 2015-03-02

Person 1 is older by 2 years, 5 months, 16 days: 899 days in total (128 whole weeks and 3 days), or 29 completed months.

A parent and a child, ending in a short month

Person 1 date of birth:: 1987-01-31
Person 2 date of birth:: 2016-04-30

Person 1 is older by 29 years, 2 months, 30 days: 10,682 days in total (1,526 whole weeks), or 350 completed months. Read this way, the gap equals the parent's age at the child's birth.

February 29 and the next February 28

Person 1 date of birth:: 2000-02-29
Person 2 date of birth:: 2001-02-28

Person 1 is older by exactly 1 year (1 year, 0 months, 0 days), because February 28, 2001 counts as the anniversary of February 29, 2000. The total is 365 days (52 whole weeks and 1 day), or 12 completed months.

The younger person entered first

Person 1 date of birth:: 2001-07-21
Person 2 date of birth:: 1998-12-05

Person 2 is older by 2 years, 7 months, 16 days: 959 days in total (137 whole weeks), or 31 completed months. Entering the dates the other way round gives the same gap.

How to Use the Age Difference Calculator

1Pick or type the first person's date of birth in Person 1 date of birth.
2Add the second person's date of birth in Person 2 date of birth. Either person can be the older one.
3Press Show Age Difference.
4Read the gap in years, months and days, then the totals beneath it. Expand Step-by-step calculation for the working, or use Clear to compare two new dates.

How the Age Difference Is Counted

The count starts on the earlier date of birth and moves forward to the later one. First come the whole years, then the whole months that fit after them, and finally the days that are still left.

Every month keeps its real length of 28, 29, 30 or 31 days; nothing is averaged. Swapping the two dates gives exactly the same gap. Only the line saying who is older changes.

Total Days, Weeks and Completed Months

Total days is the number of calendar days you step through to get from the earlier date to the later one. Total weeks splits that number into whole weeks plus any days left over.

Completed months is the number of whole calendar months in the gap: the years multiplied by 12, plus the extra months. It is not the total days divided by 30. From January 31 to April 30 there are 89 days but only 2 completed months, because the third month is not finished until May 1.

When a Birthday Falls at the End of a Month

A month is counted only when the same day number comes round again. If the later month has no such day, for example a 31st in a 30-day month, that month is completed on the 1st of the month after it.

So January 31 to February 28 is 28 days with no whole month, and January 31 to March 1 is 1 month and 1 day. A day number that every month has, such as the 15th, never needs this rule.

Birthdays on February 29

February 29 appears only in leap years. Any year that divides evenly by 4 has one, but a century year qualifies only if it also divides evenly by 400, so 2000 had a February 29 and 1900 did not.

When the earlier date is February 29 and the later year has no February 29, the calculator treats February 28 as the anniversary, the same rule the Age Calculator follows. That is why February 29, 2000 and February 28, 2001 are shown as exactly 1 year apart even though only 365 days separate them. Measured to February 29, 2004, the gap is 4 years and 1,461 days.

Past and Future Dates

Both dates can be in the past, in the future, or one of each, because the calculator compares the two dates with each other and never with today's date.

Any date from January 1 of year 1 to December 31, 9998 is accepted. The Gregorian leap-year rule is applied to every year, including years before the Gregorian calendar was introduced in 1582, which is also how dates are defined for web date fields.

Ways to Use an Age Gap

Compare two siblings, two friends or a couple, or check how far apart two historical figures were born.

For a parent and a child, the gap doubles as the parent's exact age on the child's date of birth.

To find one person's age today or on a chosen date, use the Age Calculator. To add or subtract years, months, weeks or days from a date, use the Date Calculator.

FAQ

How is the age difference between two people worked out?

Begin at the earlier of the two birth dates and count toward the later one. Take off as many whole years as fit, then as many whole months as fit after those, and what is left is the number of days. The calculator does exactly this on the real calendar and also converts the result into total days, weeks and completed months.

Does it matter which date of birth I enter first?

No. The calculator decides which date is earlier and always counts from that one, so entering the two dates the other way round gives an identical age gap. Only the line telling you who is older changes.

Why can a gap that ends in a short month show fewer months than I expect?

A month only counts once the same day number arrives again. Starting from January 31, February and April have no 31st, so each of those months is only finished once the next month begins. January 31 to April 30 is therefore 2 months and 30 days rather than 3 months.

How are February 29 birthdays handled?

When someone born on February 29 is compared with a date in a year that has no February 29, February 28 stands in as the anniversary. So February 29, 2000 to February 28, 2001 is shown as exactly 1 year, even though the total is 365 days. This matches the rule used by the Age Calculator.

Can I compare dates in the future?

Yes. Either date, or both, can lie in the future, because the two dates are only compared with each other, never with today. Dates from January 1 of year 1 through December 31, 9998 are accepted.

Why are completed months not just the total days divided by 30?

Calendar months run from 28 to 31 days, so an average would drift. Completed months counts the whole calendar months inside the gap, which is the years times 12 plus the extra months, and the remaining days are reported on their own.

Are the dates I enter stored or shared?

No. The comparison is worked out in your browser, and the calculator never uploads the two dates or keeps a copy of them. Separately, the site measures overall page visits with Google Analytics, which the privacy policy explains.

Sources

Based on 4 sources.

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