Chrislaw - thanks, that's what I already did, though, and although it confirmed that there's still a valid warranty, it couldn't give me the expiration date.
I've finally just called Apple service, and they were able to give me the expiration date. She also said, to the best of her knowledge, there's no way I could have found the date myself online. So, now I know for the future - it takes a phone call. 🙂