About one in every 1,461 people — roughly 5 million worldwide — is born on February 29. They are called "leaplings," and they share a curious problem: their birthday only appears on the calendar every four years. In the other three years, when exactly do they turn a year older? The answer involves a mix of math, law, and tradition.
Why leap day exists at all
Earth takes about 365.2422 days to orbit the Sun — not a clean 365. Without correction, calendars would drift away from the seasons over time. Adding a day every four years corrects most of the drift. Skipping the leap day on years divisible by 100 (but not 400) handles the rest. So 2024 was a leap year. 2100 will not be. 2000 was.
How often leap days actually occur
A leap day occurs:
- Every year divisible by 4 (2024, 2028, 2032, ...)
- Except years divisible by 100 (1900, 2100 — no leap day)
- Unless also divisible by 400 (2000 — leap day returns)
The result: about 97 leap days per 400 years, or one every 4.13 years on average.
The legal question: when do leaplings age up?
In the United States, the rules vary by state, but common conventions include:
- February 28. The day before March 1. Many states default to this.
- March 1. The "next" calendar day after February 29 would be. Preferred in some jurisdictions.
For most practical purposes — driving age, voting age, age-of-majority, legal drinking age — the effective birthday in non-leap years is February 28 or March 1, depending on state law. A leapling born Feb 29, 2004 reached age 18 on Feb 28, 2022 or March 1, 2022 depending on which state they lived in.
Other countries have their own rules. Taiwan uses March 1. Hong Kong uses March 1. Japan uses February 28 (day-before rule). China has no consistent statute.
The cultural question: when do leaplings celebrate?
Celebration is personal, not legal. Common choices:
- February 28 — the last day of February, closest to "the day before March."
- March 1 — the first day of March, the calendar-neighbor approach.
- Both days — two parties are better than one.
- February 29 only — celebrating just every four years (the minimalist approach).
The Honor Society of Leap Year Day Babies, a real organization, suggests either Feb 28 or March 1 is fine — whatever feels right.
Age in years: the math
For a leapling born February 29, 2000, their age on April 21, 2026:
- Birthdays "celebrated": 2001, 2002, 2003, 2004 (actual leap), 2005 ... 2024, 2025, 2026. Simple year difference: 26 − 0 = 26.
- As of April 21, the date is past both Feb 28 and March 1, so the latest birthday has occurred.
- Age: 26 years.
A leapling's actual Feb 29 birthday happens in 2000, 2004, 2008, 2012, 2016, 2020, 2024, 2028 — that is 7 "real" birthdays in their first 26 years. Hence the joke "I'm only 6 leap years old."
Edge case: newborns born Feb 29
A baby born Feb 29, 2024 turns 1 on Feb 28, 2025 or March 1, 2025 by most conventions. Their actual 1st "leap-day" birthday is Feb 29, 2028 — when they are 4 years old on the calendar.
Leap year milestones to know
- Most recent: 2024 (Feb 29, 2024 occurred)
- Next: 2028 (Feb 29, 2028)
- Following: 2032, 2036, 2040...
- Skipped: 2100 (divisible by 100 but not 400)
- Restored: 2400 (divisible by 400)
If you are born February 29 in 2100, you are a very statistically unusual person — the year will not be a leap year.
Why this matters beyond parties
Legal contracts, insurance renewals, driver's license expiration dates, subscription anniversaries, warranty periods, and birthday-triggered benefits all touch the leap-day edge case. Most software handles it by defaulting to February 28 or March 1 in non-leap years, but inconsistent handling across systems can cause confusion. When specifying important dates, use ordinal rules ("60 months from contract date") rather than calendar dates when leaplings are involved.
Calculate around a leap day
Our age calculator handles leap-day births correctly — in non-leap years, it defaults to Feb 28 while still preserving the exact "last leap-day birthday" count. Leaplings can compute their age in real years, leap-year birthdays, and total days. If you know a leapling, enter their birth date and watch both numbers appear. The math is quirky — the math is also fun.