How come leap years don't occur on years divisible by 100 that aren't divisible by 400? I read this and I was surprised that years like 1900 and 1400, which aren't divisible by 400, aren't leap years, even though they are divisible by four.  I wonder when this started happening on years divisible by 100 that don't have February 29.  May you please help me out?  Thank you.
 A: That was part of the Gregorian reform of the Julian calendar previously in use. When it took effect depends on where you’re talking about. It became the law of the Roman Catholic Church in $1582$, and most Catholic countries adopted it then or very soon thereafter. England, on the other hand, didn’t adopt it until $1752$, and Greece didn’t adopt it until $1923$; for more on the various dates of adoption see this article.
The point of the oddity is that over a span of $400$ years it brings the calendar more closely in line with the astronomical year than the Julian calendar, with its leap year every fourth year.
A: There were leap years for a long time, but only since 1752 has the criteria for today's leap year started. So, 1800 would be the first year divisible by 100 and not a leap year.
A: It is a correction factor because there are not really $365$ days in a year.  A "simple" correction of one leap year every $4$ years is not accurate enough so they came up with a better system.  Personally I think the whole calendar system is a "hack" and could have been made a lot better.
