Leap forward at both ends in three years, and leap forward at both ends in three years means leap forward every other year or leap forward in two years. Jumping at both ends in three years and not jumping at both ends in three years is what people often say: one year in three years is a leap year, two years before and after three years are leap years, and every other year is a leap month or every two years. The establishment of leap year is to make up for the time difference between the number of days in a year and the actual period of revolution of the earth caused by man-made calendar regulations. The year to make up for the time difference is leap year. The leap year has 366 days (February 1- 1 day, 29 days, 3 1 day, 30 days, 3 1 day, 3 1 day.