code: def certain_timezone_at(self, *, lng, lat):
          
  
          lng, lat = rectify_coordinates(lng, lat)
          shortcut_id_x, shortcut_id_y = coord2shortcut(lng, lat)
          possible_polygons = self.polygon_ids_of_shortcut(shortcut_id_x, shortcut_id_y)
  
          # x = longitude  y = latitude  both converted to 8byte int
          x = coord2int(lng)
          y = coord2int(lat)
  
          # check if the point is actually included in one of the polygons
          for polygon_nr in possible_polygons:
  
              # get boundaries
              self.poly_max_values.seek(4 * NR_BYTES_I * polygon_nr)
              boundaries = self.fromfile(self.poly_max_values, dtype=DTYPE_FORMAT_SIGNED_I_NUMPY, count=4)
              if not (x > boundaries[0] or x < boundaries[1] or y > boundaries[2] or y < boundaries[3]):
  
                  outside_all_holes = True
                  # when the point is within a hole of the polygon this timezone doesn't need to be checked
                  for hole_coordinates in self._holes_of_line(polygon_nr):
                      if inside_polygon(x, y, hole_coordinates):
                          outside_all_holes = False
                          break
  
                  if outside_all_holes:
                      if inside_polygon(x, y, self.coords_of(line=polygon_nr)):
                          return timezone_names[self.id_of(polygon_nr)]
  
          # no polygon has been matched
          return None
docs: this function looks up in which polygon the point certainly is included
          this is much slower than 'timezone_at'!
          :param lng: longitude of the point in degree
          :param lat: latitude in degree
          :return: the timezone name of the polygon the point is included in or None