Leap Years Explained: How They Affect Your Age and Birthday

Published on July 25, 2025

Every four years, we add an extra day to our calendar: February 29th. This day, known as a leap day, is a curiosity that can make age calculation slightly more complex. But why do we have leap years, and what does it mean for those born on this unique date?

Why Do We Need Leap Years?

The need for leap years comes down to a simple astronomical fact: the Earth's orbit around the Sun isn't a perfect 365 days. It actually takes approximately 365.2422 days for the Earth to complete one full revolution. This extra quarter of a day (roughly 6 hours) might not seem like much, but over time, it adds up.

If we used a strict 365-day calendar, our seasons would slowly drift. After 100 years, the calendar would be off by about 24 days. Summer would eventually start in what we now consider autumn, and winter would creep into spring. To keep our calendar synchronized with the Earth's orbit and the seasons, we need to periodically add an extra day.

The Rules of the Leap Year

The system for determining a leap year was established by the Gregorian calendar, which we use today. The rules are as follows:

  1. A year is a leap year if it is divisible by 4.
  2. However, if the year is divisible by 100, it is NOT a leap year...
  3. Unless the year is also divisible by 400.

This is why the year 2000 was a leap year (divisible by 400), but the years 1700, 1800, and 1900 were not (divisible by 100 but not by 400).

What About "Leaplings"?

People born on February 29th are often called "leaplings" or "leapers." They face a unique situation when it comes to celebrating their birthday. In non-leap years, there is no February 29th. Most leaplings choose to celebrate their birthday on either February 28th or March 1st.

Legally, their official age still increments each year. In most legal systems, a person born on February 29th legally attains the next age on March 1st in non-leap years. Our age calculator automatically handles these rules, so whether you're a leapling or not, you can be sure you're getting an accurate calculation of your age in years, months, and days.