I was writing code not long ago to determine if a given day is a holiday. I didn't want to use a database of dates that would have to be updated in the future. 
I was breezing along with the variable-date holidays like Thanksgiving, and then I came to Easter.<p>Turns out, it's slightly complex. It has to do with moon cycles.<p>After wigging out a little about this task, an afterthought that should've been easy, I googled for the specific formula so I could begin to express it in code. 
(I had already done a 20, 30 minute search for an existing PHP Class to handle all dates and was unsuccessful.)<p>What I found next nearly blew my mind. <a href="http://php.net/manual/en/function.easter-date.php" rel="nofollow">http://php.net/manual/en/function.easter-date.php</a><p>This at once delighted and disgusted me. A perfect example of the utility AND horror of PHP's large standard library.