Peter, what you want to do is included in the Meeus book. Peter Duffet-Smith's book Practical Astronomy with Your Calculator also covers it.

But think about what you're trying to do. Add/subtract dates...with varying length months, leap years, etc. Not to mention the different norms for notation (mm/dd/yy, mm/dd/yyyy, dd/mm/yy, etc.). Time isn't much better. While not difficult to calculate, it isn't exactly trivial either.

One big benefit from doing it yourself is understanding what's going on, thus making it easier to use in different calculations. Using someone else's code, unless it's use is documented very well, can sometimes be quite the headache.

Dennis