I had this happen to me last year. I made a reservation using the certificate and I asked if it would still be valid prior to making the reservation. I was told it would, since I was making the reservation prior to the expiration of the certificate.

Just prior to my travel date, I called again to make sure it was valid and was told it was not. I was told to send a letter to Marriott. I did so, and received another certificate good for an additional six months.

Moral of the story--use the certificate before it expires. I was lucky.