It is a little$-$known fact that not every fourth year is a leap year. Most people only ever need to learn that if a year is divisible by four, then it is a leap year. This, however, is actually a simplification of the leap year rules. Since the Earth orbits the Sun in $365\mathrm{.}2425$ days, not $365\mathrm{.}25$ days, additional leap year rules exist to correct for these additional decimal places.

In short, a year is a leap year if it is after $1582,$ and divisible by $400,$ or divisible by $4$ but not divisible by $100.$

What this means is that the year $2000,$ a leap year, was not a leap year simply because $2000$ is divisible by four, but because it is also divisible by $400!$ The year $2100,$ therefore, will not be a leap year.