docs: def duration_outside_nwh(
              self,
              starttime: datetime.time = datetime.time(NORMAL_DAY_START_H),
              endtime: datetime.time = datetime.time(NORMAL_DAY_END_H)) \
              -> datetime.timedelta:
          """
          Returns the total duration outside normal working hours, i.e.
          evenings/nights, weekends (and Bank Holidays).
          """
          total = datetime.timedelta()
          for interval in self.intervals:
              total += interval.duration_outside_nwh(starttime, endtime)
          return total