Great text on why it's so difficult to put into code human activities. Do you want a library to deal with complex math equations? No problem, you can define clear unit test that will stand the trial of time. Do you want to code time zones or addresses? Thought luck.
At my first job we had an home made CalDAV server. One feature is the Free/Busy letting people ask for other people availability. Once you try to handle 'exceptions to recurring calendar events with a side of timezones and DST' you know why you should use existing libraries.
