> 4May-1May is meaningful to me

Great, but it isn't addition.

> and frankly I do believe that it is meaningful to you.

Well you're wrong.

Consider 1Mar-28Feb: Should this be 1 or 2?

This proves it can't be subtraction (in the arithmetic sense) either.

> You might want to look in a dictionary and see what `subtract` means.

Subtraction is only addition if 4May-1May is equivalent to 4May+-1May which leaves you with an additional problem: What to do about negative plaindates.

> I do not think you have thought about what we are saying at all

I've probably thought about it too much at this point. You don't even know how arithmetic works, so I think if you have anything to say it will be too difficult for me to understand.