docs: def retrieve(self, request, *args, **kwargs):
          """Retrieve pzone as a preview or applied if no preview is provided."""
  
          when_param = get_query_params(self.request).get("preview", None)
          pk = self.kwargs["pk"]
  
          when = None
          if when_param:
              try:
                  when = parse_date(when_param)
              except ValueError:
                  # invalid format, set back to None
                  when = None
  
          pzone = None
          if when:
              # we have a date, use it
              pzone = PZone.objects.preview(pk=pk, when=when)
          else:
              # we have no date, just get the pzone
              pzone = PZone.objects.applied(pk=pk)
  
          # turn content list into json
          return Response(PZoneSerializer(pzone).data, content_type="application/json")