A leap year has 366 days instead of 365. The extra day is 29 February. This day can change the total number of days between two birth dates.
The Age Difference Calculator checks leap years automatically.
Simple leap-year rule
A year is usually a leap year when it can be divided by 4. But years ending in 00 have another rule. They must also be divided by 400.
Examples:
- 2024 was a leap year.
- 2028 will be a leap year.
- 1900 was not a leap year.
- 2000 was a leap year.
You do not need to remember this rule when using the calculator.
Years, months and days
The calendar answer counts full years, full months and the days left. A leap day may be inside that period, but it does not always add one to the day part shown at the end.
Compare 1 March 2023 with 1 March 2024. The calendar gap is exactly one year, even though that period contains 366 days. The extra leap day changes the total-day count, not the one-year description.
Total days
The total-day answer includes every day between the two dates. If 29 February falls inside the period, it is counted.
This is why the same number of calendar years can have a different number of total days.
For example:
- 1 March 2022 to 1 March 2023 is 365 days.
- 1 March 2023 to 1 March 2024 is 366 days.
Both gaps are one full calendar year.
If someone was born on 29 February
The date is valid only in a leap year. When comparing it with a date in a normal year, the calculator follows a clear month-end rule.
Official rules may treat a leap-day birthday differently for age limits or legal work. Always follow the rule written on the official form or notice.
Worked examples
Example 1: the leap day is inside the gap
From 28 February 2023 to 1 March 2024, the count passes through 29 February 2024. The total-day result includes that extra day.
Example 2: both dates are before the leap day
From 10 January 2023 to 10 January 2024, the period ends before 29 February 2024. The coming leap day is not part of the gap.
Example 3: a century year
The year 1900 was divisible by 4, but it was not divisible by 400. It was not a leap year. The year 2000 was divisible by 400, so it was a leap year.
Common leap-year mistakes
- Assuming every year divisible by 4 is a leap year.
- Adding one day only because a leap year appears in the list of years.
- Treating one calendar year as always equal to 365 days.
- Entering 29 February in a year where that date does not exist.
- Using a general calculator to decide an official eligibility date.
Quick test
Try dates that cross February in 2024. Then compare similar dates in 2023. The total-day answer may differ by one day because 2024 includes 29 February.
Also test 1900 and 2000. A calculator that marks both as leap years is using an incomplete rule.
Use the Age Difference Calculator