code: def gap_to_sorl(time_gap):
      
      quantity, unit = parse_ISO8601(time_gap)
      if unit[0] == "WEEKS":
          return "+{0}DAYS".format(quantity * 7)
      else:
          return "+{0}{1}".format(quantity, unit[0])
docs: P1D to +1DAY
      :param time_gap:
      :return: solr's format duration.