The rationale for this tiny date module is that since it's hard to get date manipulation right, we won't do any date manipulation at all. This limits the usability of this package.
Maybe some simple manipulation functions (as long as their limits were documented) would increase the rating. (These could be auto-loaded so as to keep the core "tiny".)
That said, once I've parsed a date into year, month and day, I don't need to manipulate it much. (One can use stringified dates to compare them.) So I wouldn't call it useless at all. It's just that the OO interface might be too much overhead for something simple.
It does fill a need though: DateTime takes a long time to load and can be like using a sports car for local errands.