code: def market_close(self, session, mins) -> Session:
          
          if session not in self.exch: return SessNA
          end_time = self.exch[session][-1]
          return Session(shift_time(end_time, -int(mins) + 1), end_time)
docs: Time intervals for market close
  
          Args:
              session: [allday, day, am, pm, night]
              mins: mintues before close
  
          Returns:
              Session of start_time and end_time