To answer this question, the 5 certificates will expire once year out from when they are issued. Once you meet the spend (which must be done within 3 months) the certificates are issued at latest when the statement where you meet the spend closes but it may be just a few days after actual spend is met. They will expire a year thereafter, so the answer is no, you cannot use them after they expire and Marriott has gotten very strict in extending expiration dates.