If you're looking for a general solution to that specific type of problem, then dateutils¹ is really useful. It deals with conversions, durations, matching, repeats, etc. It also has a clear and well defined date input format, and being a collection of command line tools it is easy to mangle it in to other tools.

¹ http://www.fresse.org/dateutils

I was also looking into this, and couldn't find a way to do it.

My guess is that it has support for time, but not for calendars. The fact that a "month" is a static length points in this direction.

Maybe there's going to be a calendar module in the future though?