Leap years are necessary to keep our calendar in alignment with the Earth’s revolutions around the sun. Since our calendar has exactly 365 days in a year, we would lose almost six hours from our calendar every year if we did not add February 29 to every fourth year. 闰年是对地球绕太阳运转所需时间和公历时间之差所作的调整。公历规定一年有365天，而实际上地球绕太阳公转一周所需的时间是365天5小时48分46秒。如果不算上这个差值的话，那么每四年我们就要少掉一天，所以才有了2月29日。