How Leap Years Work
Learn the 4, 100, and 400-year rule. Read a concrete example, examine the trade-offs, and try a small exercise to see whether the principle applies to your situation.
The core rule or calculation
- Start here. A year divisible by 4 is usually a leap year.
- Add context. Century years divisible by 100 are exceptions unless divisible by 400.
- Keep in mind. February has 29 days in leap years on the Gregorian calendar.
Work through an example
The year 2000 was a leap year because it was divisible by 400, whereas 1900 was not despite being divisible by 4. This extra century rule keeps the Gregorian calendar aligned more closely with the solar year.
The century exception is the detail worth remembering because most ordinary years are easy to classify.
Where calculations go wrong
The leap-year formula applies to the Gregorian calendar, not automatically to every historical date or calendar system. Software parsing older dates may also follow specific conventions.
Try the method
Test 2024, 2100 and 2400 using the 4/100/400 rules. Explain aloud why a simple “divisible by four” shortcut fails for one of them.
Open a related BlogJD calculator →
Further reading and verification
Read the linked reference for additional background on time & numbers. Review the applicable rules and update dates before using any example in a consequential decision.
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Published October 9, 2026 · BlogJD Editorial Desk · How we prepare and correct content · Report a correction.