I still think plus is fine in this case. And really, my expectation is, that anybody who thinks about adding a months, knows about their own intention. Because I suspect just adding the month is not what most people are actually doing, I guess in 90% of the cases most developers will add another step, rounding to the last (or first) day, or to the next same weekday, whatever.

The one thing nobody wants, is to add a month and land in the month one over.

So even if the semantic differs between libraries for adding a month, it actually doesn't mather that much. Because for all the other cases one could imagine, most people will add days or weeks, if staying on the same day matters.