What is a leap year?

The leap year in the Gregorian calendar is stipulated as follows: the earth's revolution around the sun is called the tropic year, with 365 days at 5: 48: 46. Therefore, the Gregorian calendar stipulates that there are an average year and a leap year. The average year is 365 days, which is 0.2422 days shorter than the tropic year and 0.9688 days shorter than the four-year * *, so one day is added every four years. This year, there are 366 days, which is a leap year. However, an increase of one day in four years is 0.03 12 more than that in four tropic years, and it will be 3. 12 more in 400 years. So there are three leap years missing in 400 years, that is, there are only 97 leap years in 400 years, so the average length of the Gregorian calendar year is similar to that of the tropic year. Therefore, it is stipulated that leap years must be multiples of 400 if they are integers. For example, 1900 and 2 100 are not leap years.