with all the certificates I've earned in the past, I had to complete the stay before the expiration date. I do remember, however, at least one post where an Insider called MR Customer Service and was able to get the expiration date of the certificate extended. Good luck. And enjoy your stays!
Hi - I have been able to extend my free night certificate each time I receive it. The customer service line will tell you they are "requesting" the extension, but it has always been granted in my case for one 6 month increment. Just call and ask, otherwise you will never know!