Every four years, our calendar adds an extra day: February 29. But why does this happen, how does the Gregorian 400-year rule work, and what happens to someone born on Leap Day when counting their exact age?
1. The Astronomical Solar Year
The Earth takes approximately 365.2422 days (365 days, 5 hours, 48 minutes, and 45 seconds) to complete one orbit around the Sun. If we used an exact 365-day calendar without adjustments, our calendar would drift by about 1 full day every 4 years. Over a single century, the seasons would shift by nearly 25 days.
2. The 3 Rules of the Gregorian Calendar
To maintain seasonal stability, Pope Gregory XIII introduced the Gregorian calendar reform in 1582 with a precise 3-rule formula:
• Rule 1: A year is a leap year if it is evenly divisible by 4 (e.g. 2020, 2024, 2028).
• Rule 2 (Century Exception): If the year is divisible by 100, it is NOT a leap year (e.g. 1900, 2100 were/will not be leap years).
• Rule 3 (400-Year Exception): If the century year is also divisible by 400, it IS a leap year (e.g. the year 2000 was a leap year, 2400 will be).
3. How Age is Counted for Leap Day Babies
Under international legal standard (such as English common law tradition followed in most jurisdictions), a person born on February 29 legally reaches the age of majority on March 1 during a non-leap year. In leap years, their birthday is observed on February 29.
Our Age Calculator incorporates this exact standard, ensuring that your completed years, months, and days remain 100% mathematically and legally coherent.