Divisible by 4?
If no, it is not a leap year.
year / 4Date and time guide
Leap-Year Birthdays Explained
A 29 February birthday only appears in leap years. In other years, people usually use either 28 February or 1 March.
The short answer
Leap-Year Birthdays Explained
A leap year adds 29 February to the calendar. Most years divisible by 4 are leap years, but century years must also be divisible by 400. That is why 2000 was a leap year and 1900 was not.
People born on 29 February still age every year. In years without 29 February, people often use either 28 February or 1 March, so a calculator should show the choice.
- Leap years have 366 days.
- Century years need the 400-year test.
- Leap-day birthdays do not make people age more slowly.
- In non-leap years, the chosen birthday rule can depend on the purpose.
Start with the method
The calculation at a glance
These are the ideas to keep separate before working through an example.
Divisible by 100?
If no, a year divisible by 4 is a leap year.
year / 100Divisible by 400?
Century years are leap years only if this test passes.
year / 400Choose the birthday date
In a non-leap year, choose 28 February or 1 March and show that choice.
Calendar rule
How to tell whether a year is a leap year
The Gregorian rule is simple once you split it into checks. Years divisible by 4 are usually leap years. Years divisible by 100 are not leap years unless they are also divisible by 400.
So 2024 and 2028 are leap years. The year 1900 fails the 400-year test, so it was not leap. The year 2000 passes it, so it was leap.
leap year = divisible by 4 and (not divisible by 100 or divisible by 400)2000 passes the 400-year test. 1900 does not.
Why it exists
The extra date keeps the calendar aligned
A solar year is not an exact whole number of days. A calendar made only of 365-day years would gradually drift relative to the seasons.
The Gregorian pattern places 97 leap days in each 400-year cycle. Its average calendar year is 365.2425 days, closely matching the seasonal year described by the U.S. Naval Observatory.
Age versus date
People born on leap day age every year
Age counts time, not how often the exact date 29 February appears. Someone born in 2000 reaches age 26 in 2026, not age six.
The tricky part is choosing the birthday date in a year with no 29 February. That choice can change a birthday countdown by one day.
Two conventions
28 February or 1 March?
Using 28 February treats the anniversary as the final day of the birth month. Using 1 March treats it as the day after 28 February, preserving the point after a full ordinary February has elapsed.
People, software and organisations may choose differently. This site defaults to 28 February for general planning and exposes a 1 March option wherever the result changes.
Calculator effects
What changes when the policy changes
Near the end of February, the selected policy can change whether the birthday is today, whether it has passed, the countdown, the weekday and the age shown at the next occurrence.
Away from that boundary, both policies may point to the same next birthday year while differing by one day. The selected convention is included in calculator output so the answer can be interpreted correctly.
Official uses
Do not assume one legal rule
Legal age and eligibility rules vary by place and purpose. A birthday date used for a party or calendar reminder may not be the rule used by a court, licence, benefit, school or service.
When the answer affects rights, deadlines or eligibility, use the wording from the responsible authority. A general birthday calculator cannot decide that for you.
Keep this handy
Leap-year examples
Apply all three divisibility tests, especially for century years.
| Year | Divisibility result | Leap year? |
|---|---|---|
| 1900 | Divisible by 100, not by 400 | No |
| 1996 | Divisible by 4, not a century | Yes |
| 2000 | Divisible by 400 | Yes |
| 2026 | Not divisible by 4 | No |
| 2028 | Divisible by 4, not a century | Yes |
| 2100 | Divisible by 100, not by 400 | No |
Follow the calculation
Worked examples
Each example uses the same calendar rules as the calculator.
1
28 February policy
Born 29 February 2000; anniversary in 2026
Use 28 February 2026- Identify 2026 as a common year.
- See that 29 February is unavailable.
- Clamp to February's final valid day.
This is the default WorkItOut.Tools planning convention.
2
1 March policy
Born 29 February 2000; anniversary in 2026
Use 1 March 2026- Identify 2026 as a common year.
- Choose the after-February convention.
- Place the anniversary on 1 March.
The calculator labels this choice rather than treating it as universal.
3
A leap year
Born 29 February 2000; anniversary in 2028
Use 29 February 2028- 2028 is divisible by four.
- It is not a century exception.
- The original month and day exist.
Both common-year policies converge on the real leap date.
Check your working
Common mistakes and better checks
Most disagreements come from an unstated convention or an incomplete shortcut.
Calling every fourth year leap
Remember the century exception and the divisible-by-400 restoration.
Dividing age by four
Leap-day births age through ordinary years just like every other birth date.
Hiding the common-year policy
A calculator should state whether it uses 28 February or 1 March.
Presenting a common choice as law
Official rules depend on place and purpose. General calendar copy should not claim one answer works everywhere.
Check another date
Use the calendar method automatically
The calculator applies the same rules while keeping your entered dates in the browser.
Open the birthday calculatorRelated tools
Use the right age calculator
When you have dates ready, these calculators apply the same calendar rules automatically.
Related guides
Keep reading the age cluster
These supporting pages cover the neighbouring calendar questions without mixing their search intent.
References
Sources and calendar conventions
These references support the calendar facts and definitions used on this page. Calculator-specific choices are disclosed separately.
- U.S. Naval Observatory: Leap YearsOfficial explanation of the leap-year rule, including century years.
- U.S. Naval Observatory: Introduction to CalendarsClear background on calendar months and leap years.
FAQ
Leap-Year Birthdays Explained questions
Clear answers for the details that most often change the result.
How do leap years work?
A Gregorian year is leap when divisible by four, except century years are leap only when also divisible by 400.
Was 1900 a leap year?
No. It is divisible by 100 but not by 400.
Was 2000 a leap year?
Yes. Although it is a century year, it is divisible by 400.
Do people born on 29 February age every four years?
No. They age continuously and complete another year every year.
Is a leap-day birthday on 28 February or 1 March?
Both are used as conventions in common years. The appropriate choice depends on the purpose and any governing rule.
Which policy does this site use?
The default is 28 February, with a visible 1 March option wherever the result can change.