Testing time/date-specific functions can be complicated. Unfortunately, because of this, many companies don’t bother testing these functions. This chapter will help you understand why testing these functions is critical, and how you can go about getting it done.
In C++ (and I think python) at least, you have access to pretty nice date_time classes in the form of Boost. It supports the setting of dates, time based math, intervals, etc, all in a nice, relatively easy to use package. If you have to work with dates, and are using C++, I would definitely give boost a look. It will likely save you lots of time.
-D