How Many Days Are in a Leap Year?

A practical method

A leap year has 366 days because February contains 29 days. The Gregorian calendar adds that day to keep calendar seasons aligned with Earth's orbit. The rule is more precise than 'every four years' because most century years are excluded.

1. Check divisibility by 400: If the year is divisible by 400, it is a leap year.
2. Otherwise check divisibility by 100: A century year not divisible by 400 is not a leap year.
3. Otherwise check divisibility by 4: A non-century year divisible by 4 is a leap year.
4. Apply the extra day: Dates from March onward have a day-of-year value one higher than in a common year.

Worked example

2000 was a leap year because it is divisible by 400. The year 2100 will not be a leap year because it is divisible by 100 but not by 400. The year 2028 is a leap year because it is divisible by 4 and is not a century year.

Checks, edge cases, and common mistakes

• Do not assume every year divisible by 4 is a leap year.
• Use the calendar system appropriate to the historical date.
• February 29 affects date ranges, age calculations, and day-of-year values.
• Validate imported dates because February 29 is invalid in a common year.