Leap year

Question

Why we always have to modulus the leap year by 100 and 400

Answer 1

It is basically due to some mathematical calculations.

IN DETAIL:
There is a leap year every year whose number is perfectly divisible by four - except for years which are both divisible by 100 and not divisible by 400. The second part of the rule effects century years. For example; the century years 1600 and 2000 are leap years, but the century years 1700, 1800, and 1900 are not.