No idea, never had it come up personally, but if I were concerned I'd just ask my "sponsor" for a new form.

In fact, in that situation, holding both forms, I might first offer the expiring one just to gather the data point since I'd have the new one in reserve if necessary.

Although this also seems like the sort of thing that might be up to the interpretation of the individual agent checking you in? (I think they're supposed to key in the code from the form but dunno if the computer does any date lookup/validation as a result of that... given the state of Marriott IT, I would tend to doubt it)