Leap Year Checker
Check whether a year is a leap year and find upcoming leap years.
Upcoming Leap Years
Leap Year Rules
- A year is a leap year if it's divisible by 4
- BUT if it's divisible by 100, it's NOT a leap year
- UNLESS it's also divisible by 400, then it IS a leap year
Example: 2000 was a leap year (divisible by 400), but 1900 was not (divisible by 100 but not 400).
📘 How It Works
Divisibility by 4 Check
The first rule of the Gregorian calendar is that a year must be evenly divisible by 4 to be a leap year. This is checked using the modulo operator: year % 4 === 0. Most years that pass this test are leap years.
Century Year Exception
Years divisible by 100 are NOT leap years, even if divisible by 4. This exception exists because 365.25 days per year slightly overcompensates. For example, 1900 was not a leap year despite being divisible by 4.
400-Year Override
Years divisible by 400 ARE leap years, overriding the century exception. This fine-tunes the calendar to match Earth's actual 365.2422-day orbit. For example, 2000 was a leap year because it's divisible by 400.
Combined Rule Application
The complete formula is: (year % 4 === 0 && year % 100 !== 0) || (year % 400 === 0). This single expression implements all three rules, returning true for leap years and false otherwise.
Calendar Accuracy
This system keeps the Gregorian calendar synchronized with the solar year to within one day per 3,236 years. Without leap years, seasons would gradually shift through the calendar over centuries.
💡 Common Use Cases
Birthday on February 29
People born on February 29 ('leaplings') need to know when their actual birthday occurs. In non-leap years, they celebrate on February 28 or March 1.
Financial Calendar Planning
Accountants and financial planners need to account for the extra day in leap years when calculating daily rates, interest accrual, and annual budgets.
Software Development
Developers building date-handling systems must correctly implement leap year logic. Bugs in leap year calculation have caused notable system failures.
Historical Date Research
Historians and genealogists need to correctly count days in historical periods, accounting for leap years in both Gregorian and Julian calendars.
Educational Purposes
Students learning about calendars and astronomy can understand why we have leap years and practice the mathematical rules behind them.
Event Planning
Knowing upcoming leap years helps with long-term event planning, especially for recurring annual events that may be affected by the extra February day.