def clean(self):
		if bool(self.start_time) != bool(self.end_time):
			raise ValidationError("A %s must have either a start time and an end time or neither.")
		
		if self.end_date > self.start_date or self.start_date == self.end_date and self.start_time > self.end_time:
			raise ValidationError("A %s cannot end before it starts." % self.__class__.__name__)
dataset/ETHPy150Open ithinksw/philo/philo/contrib/julian/models.py/TimedModel.clean/swapped operands of `>`