leap year was established to make up for the time difference between the annual number of days caused by man-made calendar regulations and the actual revolution period of the earth. the year in which the time difference is made up is a leap year. there are 366 days in a leap year (31 days, 29 days, 31 days, 30 days, 31 days, 30 days, 31 days, 31 days, 30 days, 31 days, 30 days, 31 days respectively from january to december).