Well, a "month", unqualified, is not really a unit of time. You have to know not only which month you're talking about, but in the case of February, what year it's in, to know exactly how long it is. The same goes for years, if you want them to consist of an integral number of days.

So I think as a matter of API design, Ruby has made a wrong choice by making these operations look like arithmetic on known values. It's too tempting to think that they'll obey the arithmetic identities, when they don't. If you want to have an API function "same date N months later", I have no problem with that at all; then it's much less tempting to think that it's just doing addition.