Personally I think that is completely unreasonable though. If you are going to give a grade you have no right to shorten the time arbitarily. I can't imagine someone saying before a university exam - sorry we are giving you less time than usual because we want an early lunch. If there was, say a 8.2 earthquake half way through the exam I'm guessing they would just give everyone another chance or scale the marks (assuming the building didn't collapse and kill everyone).