Marriott's system doesn't check to verify that the cert will be valid during the stay, only that it is currently valid when linked. Thus you can make a reservation today with a cert that is currently good, but will expire before the stay.

But, when that cert expires, it will no longer be linked to that stay. You will then either need to have another cert linked or use points to order a cert. I do believe Marriott's system will automatically pull the points from your account (about a week before the stay) if you haven't linked another valid cert to the reservation.