Calendar Rules & Leap Years
Understanding calendar systems, leap year rules, and the mathematics behind our date system.
Why & What
Our calendar exists because Earth's orbit around the Sun takes approximately 365.2422 days—not an exact number. This fractional day must be accounted for, or our calendar would slowly drift out of sync with the seasons.
The Gregorian calendar (used worldwide today) was introduced in 1582 to correct errors in the older Julian calendar. It uses a clever system of leap years to keep the calendar aligned with Earth's orbit.
Leap Year Rules
A year is a leap year if:
- 2024: Divisible by 4 → Leap Year
- 1900: Divisible by 4 and 100, but not 400 → Not a Leap Year
- 2000: Divisible by 4, 100, and 400 → Leap Year
- 2023: Not divisible by 4 → Not a Leap Year
Leap Year Checker
Days in Month Calculator
Days in Each Month
31
28/29
31
30
31
30
31
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31
Mnemonic: "30 days has September, April, June, and November. All the rest have 31, except February alone, which has 28 days clear, and 29 in each leap year."
Other Calendar Systems
Introduced by Julius Caesar in 45 BCE. Leap year every 4 years (no 100/400 exception). Drifted by about 10 days by 1582.
Lunisolar calendar with 12 or 13 months. Used for traditional festivals. Years are named in 60-year cycles.
Lunar calendar with 12 months of 29 or 30 days. Year is about 11 days shorter than solar year.
Note
- Gregorian calendar rules apply from 1582 onwards.
- Different countries adopted the Gregorian calendar at different times.
- Earth's rotation is gradually slowing, requiring occasional "leap seconds."