Suppose you wanted to provide a basic interface where users could set training goals, stuff like "Run 50km in June" perhaps. This seems semi-reasonable right? - well, at least as long as nobody ever wants to set a goal for two weeks.
So surely if I want a goal for one month I can just set Calendar month, pick a day in the month (or maybe it'll even flip to be a month dropdown?), and go, right?
Not so fast bucko, UI has got to be validated and you jackasses aren't going to start putting goals for the past into our system!
Clearly the designer of this never had to actually use it! Arglefarg.