Leap year most years that are multiples of 4 are leap years. In each leap year, the month of February has 29 days instead of 28. Adding one extra day in the calendar every four years compensates for the fact that a period of 365 days is shorter than a tropical year by almost 6 hours. Some exceptions to this basic rule are required since the duration of a tropical year is slightly less than 365.25 days. The Gregorian reform modified the Julian calendar's scheme of leap years as follows: Over a period of four centuries, the accumulated error of adding a
Leap year leap day "every" four years amounts to about three extra days. The Gregorian calendar therefore drops three leap days every 400 years, which is the length of its "leap cycle". This is done by dropping February 29 in the three century years (multiples of 100) that cannot be exactly divided by 400. The years 1600, 2000 and 2400 are leap years, while 1700, 1800, 1900, 2100, 2200 and 2300 are common years. By this rule, the average number of days per year is 365 + − + = 365.2425. The rule can be applied to years before the Gregorian reform