function get_date_string_from_sweep,sweep

    date_str = $
      string(sweep.ray[0].h.year,format='(i4.4)') + '_' + $
      string(sweep.ray[0].h.month,format='(i2.2)')      + $
      string(sweep.ray[0].h.day,format='(i2.2)') + '_'  + $
      string(sweep.ray[0].h.hour,format='(i2.2)')       + $
      string(sweep.ray[0].h.minute,format='(i2.2)')     + $
      string(sweep.ray[0].h.sec,format='(i2.2)')
    return,date_str
    end