unknown. I've gotten three sets several months before the second year elapsed. The second set was only a single certificate which was recovered from Chase after losing the first pair of certificates on a flight that never happened. The third set was a pair that showed up about 7 months into the second year.
There's quite a few restrictions on mine so they are a bit hard to use. Round-trip travel is required. Co-terminals are not recognized. SFO-CLE + CLE-SJC does not count as a round-trip. It's too bad that I can't use them on international itineraries either.