It has an expiration date for a reason. It probably won't be hardened in the tube, but like others have said, when you use it it may not cure - ever. But you'll only have that happen once, trust me. It is a *huge* PITA to remove uncured silicone, clean all the surfaces, and start over. After that you'll never use expired silicone again.
If it was a few weeks expired it'd probably be ok. But a year? You wouldn't drink milk that had been expired for a year, would you?
Honestly, I won't use expired silicone, period. I had one run-in with it on a building project (not pet related), and that was enough to make me pay careful attention to the expiration date. I now throw away tubes after they expire to make sure I don't use them as well.