code: def wait_time(departure, now=None):
      
  
      now = now or datetime.datetime.now()
      yn, mn, dn = now.year, now.month, now.day
      hour, minute = map(int, departure.split(':'))
      dt = datetime.datetime(yn, mn, dn, hour=hour, minute=minute)
      delta = (dt - now).seconds
      if (dt - now).days < 0:
          delta = 0
      if delta < 3600:
          return '%02d:%02d' % (delta // 60, delta % 60)
      else:
          delta_hh = delta // 3600
          delta_rest = delta - delta_hh * 3600
          return '%02d:%02d:%02d' % (delta_hh, delta_rest // 60, delta_rest % 60)
docs: Calculate waiting time until the next departure time in 'HH:MM' format.
  
      Return time-delta (as 'MM:SS') from now until next departure time in the 
      future ('HH:MM') given as (year, month, day, hour, minute, seconds). 
      Time-deltas shorter than 60 seconds are reduced to 0.