The rule is that leap years are the ones divisible by 4. Unless it’s also divisible by 100. Unless unless it’s divisible by 400.

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

Ahh, so it's centuries that aren't divisible by 400 rather than that aren't divisible by 4, that makes more sense!

Thanks for answering

It's centuries that aren't divisible by 4. It isn't years that aren't divisible by 4.

The GP formulated it in a somewhat unclear way. "Centuries" divisible by 4 probably meant "years" divisible by 400.

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.