code: public ZoneId getAgencyTimeZoneForStop (String stop_id) {
          StopTime stopTime = getStopTimesForStop(stop_id).iterator().next();
  
          Trip trip = this.trips.get(stopTime.trip_id);
          Route route = this.routes.get(trip.route_id);
          Agency agency = route.agency_id != null ? this.agency.get(route.agency_id) : this.agency.get(0);
  
          return ZoneId.of(agency.agency_timezone);
      }
docs: Get the likely time zone for a stop using the agency of the first stop time encountered for the stop.