I want to report that I had an upcoming free night certificate whos expiration date was approaching (expiration date was 6 months from the date of issue). I was unable to use the certificate due to obligations on our calendar, but didn't want to waste the award. I contacted Marriott Rewards Customer Support at 1 801 468 4000 (8:15am - 9:00pm, Monday - Friday, US Eastern time) and asked it would be possible to receive an extension on the expiration date. Marriott graciously re-issued the certificate with an expiration date of 6 months from the date of re-issue.
Thank you, Marriott!