Over a period of four centuries, the accumulated error of adding a leap day "every" four years amounts to about three extra days. The Gregorian calendar therefore omits three leap days every 400 years, which is the length of its "leap cycle". This is done by omitting February 29 in the three century years (multiples of 100) that cannot be exactly divided by 400. The years 1700, 1800, and 1900 were not leap years; neither will 2100, 2200 and 2300. Conversely, the years 1600 and 2000 were leap years as will be 2400. 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 (the proleptic Gregorian calendar), if astronomical year numbering is used.