[This article](http://stackoverflow.com/questions/3220163/how-to-find-leap-year-programatically-in-c#11595914) explains the 100 and 400 modulo checks can be factored down to 25 and 16. [The Date algorithms](http://howardhinnant.github.io/date_algorithms.html#is_leap) page should contain this optimization.
This article explains the 100 and 400 modulo checks can be factored down to 25 and 16.
The Date algorithms page should contain this optimization.