But I do remember there being some weird niche rules about which years are or aren't leap years, so I'm guessing your comment is basically right just wrongly worded?

So 2000 was leap, but 2100, 2200, and 2300 won’t be, but 2400 will be.

Thanks for answering

So, 19th century (1900 is the last year) isn't divisible by 4 (19/4 is not integer), which is the same as saying that 1900 isn't divisible by 400.

This is the main reform of the Gregorian calendar - leap days aren't introduced on xy00 years which aren't divisible by 400. This corrects the length of a year to 365.2425 days, which is fairly close to the real value of 364.2422 days.

The original Julian calendar had year of 365.25 days, which aggregated an error of more than ten days over the centuries.