1, formula: once every four years; If you don't leap in one hundred years, you will leap again in four hundred years.
2. Calculation method:
① Divided by 4, the divisible year is generally a leap year, not a normal year. ② Like 1900, there are two zeros at the end, which is generally an average year.
③ Like 400,800, 1200 and 1600, 2000 is a leap year.
3. "Jump in four years, not in a hundred years, and jump again in four hundred years". It is inferred that 2000 is a leap year, but 2 100 is not. Similarly, "leap again in 400 years" actually lost 2 hours, 53 minutes and 20 seconds, and after 8 400 years (3200 years) lost 23 hours, 6 minutes and 40 seconds.