In the past I have had success booking for after the date on the certificate (if you know when and where you want to go)
Call them and ask.Got it extended 6 mos. then extended it again as one time courtesyGave them dates both times to make new reservation (obviously i cancelled the first one)
Was your Marriot card still open when you got it extended ? My wife's and mine for that matter have Been closed .
mine wasnt