def METHOD_NAME(self):
    # Don't allow a start date before an end date
    if self.period_start and self.period_end and (self.period_start > self.period_end):
        raise ValidationError(
            {'period_start': '%s' % _('Period start cannot be at a later time than period '
                                      'end.'),
             'period_end': '%s' % _('Period start cannot be at a later time than period '
                                    'end.')}
        )