Once it turns midnight on the night of December 31st then it is officially Jan 1, 2008. So actually both times you listed are the same date and either time would be good. But if it was me and I was the one betting the other he couldnt lose the weight....I would do the weigh in at midnight, because he was probably eating alot at a party and drinking alot of beer up until midnight while celebrating the new years eve party and might weigh alittle more than he would hours later into the day.