I think Opening Day. I thought they can go X number of days back into spring training. I think it can be backed into spring training, too. IIRC, Prior opened '05 on the DL but pitched the second week of the season (Opening Day was April 4 and he pitched vs San Diego on the 13th, I believe). It may even be able to go back into the previous season, i.e. the last time he actually appeared in a game.