Calendar bugfix, or, Why Massless Can't Add.

My normally crappy code overflowed the toilet bowl recently. A math issue. If I have any flaw as a programmer (and there are admittedly few...in fact my only flaw, really, is that I work myself too hard and have to choose between innovation and brilliance and...hey, hey, where are you going?...) well, it's that damn adding and subtracting and adding again.

Sometimes the months in the previously-mentioned calendar scripts had, um, more than 31 days. And some months other than February had 28. Yck. Special thanks to Howard van Rooijen for being gracefully critical and providing the workaround.

Good thing I'm not working on flight guidance systems, huh?