takuji31/p5-Time-Piece-Plus — pull request #1 by Songmu (2013-02-01 19:00:44+00:00)
Title: override add/subtract method for providing like DateTime interface.

I want to use add/subtract method like DateTime.

Then I overrided `add` and `subtract` method providing interface like DateTime.
These methods don't change object itself. This is difference from DateTime.

This feature doesn't break original behavior of these methods.

If you like this. Merge it.