Join Date: Jan 2008
Location: JFK, BUR, YVR
Programs: AC, AS (PLT), BA, DL, HH (G), MR (TIT/LTP), UA (*S), UScAAre (1,94MM), WN
Posts: 5,332
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."