Your questions got me interested also, so I did some searching. You might call your local Master Gardener Hotline. They get these kinds of questions all the time.
http://www.uri.edu/cels/ceoc/c...
It looks like their gardening education program is superb, and they even offer free seeds!
Take them up on it!
I'm in zone 6b on some maps, 7a on others, and one has me as 6A! From year to year, though, it really differs. I still have eggplant on the plant well into November some years.
Our Extension service has a free weekly online newsletter that is most helpful. It addresses the current weather, instead of just using past averages. You might see if you have such a thing there.