The gregorian calendar is based on the slightly less that 365.25 days in a year. So.....

Leap years ar not leap years if divisible by 100, but are if divisible by 400, which is good for the next thousand years or so,

FWIW even in the Gregorian calendar the year dot does not exist. the year 1AD (or should that be CE?) is 1BC there is no zero.

And I thought Gregorian was a game of chants.........