[ComputerGuru]

> The leap year cycle is 4 days (excluding the 100 year skips)

I think you mean “The leap year cycle is 4 days (excluding the 100 year skips, except every fourth 100 years when they don’t),” eg how 2000 was a leap year despite this individual rule.

(I know it’s accounted for elsewhere in the modulo logic, but you shouldn’t resist the impulse to point out just how crazy it all is!)

[jcranmer]

The 400 year rule is already accounted for by taking the year mod 400. 2000, 2400, 2800, etc. will all use the same calendar.

[ComputerGuru]

Yes, I did mention you had it taken care of, just not explicitly pointed out :)