Couldn't you end up parsing the two dates (which seem likely to be entered in a consistent format) in two different ways with this construct?  I'm thinking particularly of the case where one date might be ambiguous ("12/01/2014") but the other date would imply/clarify what was meant ("12/31/2014").

I don't have any particularly good ideas as to how one might handle this if they are at all likely to enter mixed formats, though.