With xLights 2012a, it is no longer necessary to hand-edit the schedule file. A playlist can be set to start or end on any minute between 00:00 and 23:59. 23:59 is treat specially as 23:59:59, so there will be no gaps in your show if you want to continue into the next day. To continue your show past midnight, simply schedule the same playlist to start at 00:00 on the next day.
Matt