system
Given the following post, respond as if you're commenting on Reddit:
human
Why is the year 2100 not a leap year?

2096 and 2104 both are, so why not that too?
gpt
Leap years make up for the fact that a year is not exactly 365 days.

Having one every four years works if the year is 365.25 days long, which is almost right, but not quite.  If we had a leap day every 4 years, then after 400 years, we'd be off by about 3 days.  So we want to eliminate 3 leap years -- so the years 2100, 2200, and 2300 won't be leap years, though 2000 was.

So the rule is this: Years divisible by 400 are leaps years.  Years divisible by 100 but not by 400 are NOT leap years.  Years not divisible by 100 but divisible by 4 are leap years.  Years not divisible by 4 are not leap years.