docs: def time_afterwards_preceding(
              self, when: datetime.datetime) -> Optional[datetime.timedelta]:
          """
          Returns the time after our last interval, but before ``when``.
          If ``self`` is an empty list, returns ``None``.
          """
          if self.is_empty():
              return None
          end_time = self.end_datetime()
          if when <= end_time:
              return datetime.timedelta()
          else:
              return when - end_time