Leap year and calendar

In short: A year lasts about 365.2422 days. To keep the calendar in step with the seasons a leap day, 29 February, is inserted every four years.

In more detail: In the Gregorian calendar valid today there are exceptions for century years.

In detail

• Rule: Years divisible by 4 are leap years.
• Exception: Century years are not leap years unless divisible by 400; 1900 was not, 2000 was.
• Julian calendar: Had only the four-year rule and therefore drifted over time.
• Gregorian reform: Introduced in 1582, days were skipped.
• Programming: Account for leap years in date calculations (zeitzonen-und-kalenderfallen).
• Birthday on 29 February: In non-leap years often celebrated on 28 February or 1 March.
• Leap second: Occasional compensation for Earth’s rotation (zeitzonen-und-datumsgrenze).