I've seen posts here by users who were able to talk Apple into allowing a purchase after the end date. I think it was only a week or two after. The users gave a sob story about being out of the country or something. Anyway, they claimed they were able to buy it and register it. I know there was more than one person claiming to have done it. They did it directly through Applecare though, not by buying it elsewhere first.
Exactly. Applecare must be bought before the expiry of the date of the one year warranty and not a day later.
As some of the other posters have pointed out, you may, as a favour, be allowed to buy it if you are a few days over the time limit, but, this is a favour, or privilege occasionally granted, not a right to be expected.