Your certificate is redeemed on the day of departure, so I would imagine the answer to your query is: "no, you cannot use expired certificates on travel booked before the expiry date."
OTOH, I haven't been able to get anyone to take my certificates on the day of travel for the last 8 or so flight segments dating back to last December. When I offered them at check-in, the TAs didn't seem to bother looking at them and didn't take them even after prompting. So for the last couple of segments I didn't pull them out and was never asked for them.
