Leap Years and Age Calculations

A leap year has 366 days instead of 365, with February 29 added to keep the calendar aligned with Earth's orbit around the sun. This extra day creates several edge cases in age calculations, date arithmetic, and software date libraries.

The leap year rule

A year is a leap year if:

1. It is divisible by 4, AND
2. Not divisible by 100 unless also divisible by 400

2000 → leap year  (divisible by 400)
1900 → not a leap year  (divisible by 100, not 400)
2024 → leap year  (divisible by 4, not 100)
2025 → not a leap year
2100 → not a leap year  (divisible by 100, not 400)

The rule produces a calendar year of 365.2425 days on average — accurate enough that the calendar will not noticeably drift for thousands of years.

The February 29 birthday problem

If someone is born on February 29, most jurisdictions have no legal standard for which day they "officially" turn a given age in non-leap years. Developers choose one of two conventions:

Convention	When Feb 29 birthday ages up
Next valid day	March 1 in non-leap years
Last valid day of Feb	February 28 in non-leap years

Neither is universally correct — the right answer depends on the applicable law (for legal age thresholds) or the product's business rules.

In practice, for age-gating features (alcohol purchase, driving licence, voting), rely on the authoritative legal rule for the jurisdiction, not a generic calculator.

How age calculations work in software

Most date libraries compute age as the number of complete calendar anniversaries:

Age = (today.year - birthdate.year) − adjustment

Where adjustment is 1 if the birthday has not yet occurred this calendar year (i.e. the month/day of today is before the month/day of birthdate), otherwise 0.

Potential bug: Some implementations compare month and day numerically and fail on Feb 29 → Feb 28 comparisons in non-leap years, off-by-one-ing the age for Feb 29 birthdays by a year.

Date arithmetic and leap years

Adding "1 year" to a date is not the same as adding 365 days:

• 2024-02-29 + 1 year = 2025-02-28 (or 2025-03-01, depending on library convention)
• 2024-02-29 + 365 days = 2025-03-01

This difference matters for recurring billing, contract renewal dates, and loan terms expressed in years.

When you need a year from today:

• Use your library's addYears() — it handles February correctly
• Avoid addDays(365) — it ignores leap years

Quick reference: recent and upcoming leap years

2020, 2024, 2028, 2032, 2036, 2040 — every 4 years through 2096, then 2104 (2100 is skipped).

Explore exact date differences including leap-year-aware day counts with the Age Calculator and Date Difference.