Why Are There 365 Days In A Year Not 364 365 d to y conversion A day is the approximate time it takes for the Earth to complete one rotation It is defined as exactly 86,400 seconds In the Gregorian calendar, a year has on average days It is based on the amount of time itThe fourth calendar year is 366 days long The average length of the calendar year in days now becomes (3 x 365 366)/4 = days 365 1/4 days is equal to