code: def day_start_ut(self, ut):
          
          # set timezone to the one of gtfs
          old_tz = self.set_current_process_time_zone()
          ut = time.mktime(time.localtime(ut)[:3] + (12, 00, 0, 0, 0, -1)) - 43200
          set_process_timezone(old_tz)
          return ut
docs: Convert unixtime to unixtime on GTFS start-of-day.
  
          GTFS defines the start of a day as "noon minus 12 hours" to solve
          most DST-related problems. This means that on DST-changing days,
          the day start isn't midnight. This function isn't idempotent.
          Running it twice on the "move clocks backwards" day will result in
          being one day too early.
  
          Parameters
          ----------
          ut: int
              Unixtime
  
          Returns
          -------
          ut: int
              Unixtime corresponding to start of day