A lot of folks have had success geting certificates extended. Give MR customer service a call. Chances are pretty good they'll accomodate you especially since you are only asking for an extention of about a week.
Welcome to the Marriott Rewards Insiders community! I see that you have already met one of our 'experts', bejacob. As he suggested, we also recommend you calling the Marriott reservations line directly at 1-800-228-9290.