For convenience, the average year is calculated as 365 days, so that every four years, 5 hours, 48 minutes and 46 seconds ×4=23 hours 15 minutes and 4 seconds are lost, which is close to one day. Therefore, one day is added in February of the fourth year, which is called leap year (366 days). Because every day of more than four years is usually in a year in which the number of AD years is divisible by 4, years like 1988 and 1992 are leap years. This is the truth of "four-year leap". Because a day is the time required for the earth to rotate once, but a day is actually 24 hours, 24 hours -23 hours 15 minutes and 4 seconds =44 minutes and 56 seconds, so it is added 44 minutes and 56 seconds every four years. Once every 400 years: 44 minutes and 56 seconds × 100=3 days, 2 hours, 53 minutes and 20 seconds. So every 400 years, three leap years should be removed. Therefore, it is stipulated that "every hundred years does not leap, and every four hundred years leap", that is, when the number of years in AD is even hundred years, although it can be divisible by 4, it cannot be divisible by 400, so it is not a leap year.