Leap Year A leap year is a year in which an extra day is added to the calendar in order to synchronize it with the seasons . Since the tropical year is 365.242190 days long, a leap year must be added roughly once every four years (four times the fractional day gives ). In a leap year, the extra day (known as a leap day ) is added at the end of February, giving it 29 instead of the usual 28 days. In the Gregorian calendar currently in use worldwide (except perhaps the Russian and Iranian calendars ), there is a leap year every year divisible by four except for years which are both divisible by 100 and not divisible by 400. Therefore, the year 2000 will be a leap year, but the years 1700, 1800, and 1900 were not. The complete list of leap years in the first half of the 21st century is therefore 2000, 2004, 2008, 2012, 2016, 2020, 2024, 2028, 2032, 2036, 2040, 2044, and 2048. The extra rule involving centuries is an additional correction to make up for the fact th...