I usually "negotiate" for the (ahem) conference at the time of hiring, not at the interview.

Besides, worse come to worse, I would just not mention it until a few weeks before and say something like: "I'm sorry, but I will need to take time off in order to settle a land dispute on Pennsylvania, as I may be required to make an appearance in court." The great thing about it is... there's absolutely nothing untruthful about making such a statement! ;o)