His ‘birthday’ that he made up on his own and with Sweet’s help corresponds with two things:

1) The landing of the Mayflower, which was on November 21st.

He remembers the date due to the fact, when he was with America, neither of them knew what a ‘date’ truly was. Al heard the people coming off the boat talking about the date, and it was the first date he ever heard and remembered it.

2) The year is based upon the year in which he first met Sweet, which was in 1650.

To further differ himself from America, he did not want to share a birthday of July 7th, but that didn’t mean he didn’t want a birthday. (He wanted what America had, after all). So with Sweet’s help he created his own birthday:

November 21st, 1650.

This is not truly accurate, he is probably a bit older and has no idea the true date, but he considers this good enough. Counting off from that date, his age would be 362.

(This would also make him a Scorpio)