There is the trade date, which is when the order is executed, and the settlement date 3 days later. For tax purposes the trade date rules. If you submit an order after the market closes 12/31/2008 and it isn't executed until 1/2/2009, it's a 2009 transaction.

Suppose you put in a limit order to sell, good until cancelled during market hours on 12/31/08 and the order does not execute on 12/31, but, does execute on 1/2/09?

Bob