code: def businesstime_hours(self, d1, d2):
          
          open_hours = self.open_hours.seconds / 3600
          btd = self.businesstimedelta(d1, d2)
          btd_hours = btd.seconds / 3600
          return datetime.timedelta(hours=(btd.days * open_hours + btd_hours))
docs: Returns a datetime.timedelta of business hours between d1 and d2,
          based on the length of the businessday