If it is the second night you need to cancel I would do as often1 says, deal with it at check in. Take your documentation that doesn't show an cancel by date. If it is the first night you need to cancel it will probably be more of a problem and not sure how you should handle that if neither the hotel or Hilton will deal with it. And if you don't show on the first night they may cancel the second night as you would be a no show for the first night and they will probably charge you money for the first night at the no show rate. So, hopefully it is the second night you need to cancel.