Quick answer
Elapsed age needs no special handling. What needs a stated rule is which date counts as the anniversary in the three years out of four when 29 February does not exist.
Two separate questions
Elapsed age is not affected by leap days at all. Time passes at the same rate for someone born on 29 February as for anyone else, and calendar-aware arithmetic counts the interval correctly without needing to know the birth date is unusual.
The question that does need a rule is anniversaries: on which date does a leapling turn a year older in a non-leap year? That is a convention, not a calculation, and different systems answer it differently.
Use this guide with the right tool: Open the Leap Year Calculator for whether a selected year follows the Gregorian leap-year rule. If the question shifts, compare it with the Age Calculator or the Birthday Countdown Calculator.
28 February or 1 March
Both conventions exist. Treating 28 February as the anniversary keeps the birthday in the same month and is what most calendar-safe date arithmetic does by default, including the calculators here. Treating 1 March as the anniversary reflects the reasoning that the person has completed a full year only once 28 February has passed.
Different jurisdictions take different views for legal purposes, and the answer can vary by what the rule is about — driving age, drinking age, majority, insurance, contract terms. Where a threshold matters, find the specific rule rather than assuming either convention.
Frequency and the century exception
A leap-day birthday recurs every four years, with one exception: century years are leap years only when divisible by 400. So 2000 was a leap year and 1900 was not, which means anyone born on 29 February 1896 waited eight years for their next actual birthday.
The next such gap is 2100, which will not be a leap year. Someone born on 29 February 2096 will not see 29 February again until 2104. It is a curiosity rather than a practical problem for anyone alive now, but it is the reason the divisible-by-four shortcut is wrong.
Everyday consequences
Forms and systems that reject 29 February as an invalid date are still surprisingly common, and the usual workaround is to enter 28 February or 1 March — which then propagates as the recorded date of birth. If that has happened, the mismatch will surface eventually against a document that has the real date.
For celebration purposes the whole thing is a matter of preference, and many leaplings mark both non-leap dates or alternate. The only thing worth being consistent about is anything with a record attached.
Worked example
Numbers from a worked example are illustrative, not precedent. Run yours, keep the inputs beside the result, and confirm any rule that carries a consequence.
Common mistakes to avoid
- Adding 365 days per year. This drifts by about a day every four years. Add calendar years and let the arithmetic handle leap days.
- Assuming one anniversary convention is universal. 28 February and 1 March are both used, and the answer can vary by what the rule governs.
- Using the divisible-by-four shortcut. Century years need to be divisible by 400. 1900 was not a leap year; 2000 was.
Use the right calculator
These cover the calculations described above. Every page shows its method, so a result can be checked rather than taken on trust.
Frequently asked questions
When does a leapling legally turn 18?
It depends on the jurisdiction and sometimes on what the rule concerns. Find the specific rule rather than applying a general convention.
Which convention does this site use?
Calendar-safe arithmetic, which clamps to 28 February. The 1 March convention is noted where relevant so you can see the difference.
How many people are affected?
Roughly one birth in 1,461, which is a few million people worldwide — enough that systems rejecting the date is a real problem rather than an edge case.