Just thought I'd share my experience / datapoint. Had 2 certs expiring before I could actually use them (Megabonus expring 5/1 and the Cardmember Anniversary one expiring 3/31). Trying to book a few nights in different places in late May.
I called in the reservation line saying i was having trouble using a certificate on a certain date. (past the expiration) Rep explained that they have to be used before they expire, not just booked.
I asked if there was any way to extend? Rep transferred me to Rewards department.
Explained the situation that I'm trying to book hotels today using those certs.
Rep was super nice and offered to extend them for me without even asking.
She was able to extend the Megabonus one for 6 months from today, Anniversary one for 1yr .

If it matters I'm Gold Elite (just barely made it for first time last year) and currently still have the CC.