Towards the end of the episode after the party Brandon indicates he's beat and going to bed. The party seemed to start just as it was getting dark and would most likely have lasted several hours. If the episode was based around the same time of year as it aired or even if it is assumed the party was during Daylight Savings, the party would have started anywhere between 6:00 and 8:00 P.M. The clock on the mantle appeared to read about 6:25 when the Walsh family comes in from the party.