Leap Year Rule

The Gregorian calendar adds an extra day to February in most years divisible by 4 so the calendar stays aligned with Earth’s orbit around the Sun, which is about 365.2422 days long. However, years divisible by 100 are skipped because adding too many leap days would make the calendar drift too far; years divisible by 400 are added back in to correct that adjustment. This rule gives 97 leap years in every 400-year cycle, which matches the solar year very closely.

Use this rule when checking whether a specific Gregorian year has 365 or 366 days. It is common in date calculations, calendar programming, scheduling, age/time problems, and any question involving February 29.

• Thinking every year divisible by 4 is a leap year, including century years like 2100
• Forgetting the special exception for years divisible by 400
• Using the rule for the Julian calendar instead of the Gregorian calendar