HOW TO GET QUARTRLY START AND END DATE ?

/* Formatted on 2013/06/17 12:35 (Formatter Plus v4.8.8) */
SELECT     ADD_MONTHS (TRUNC (param.start_date, 'Q'),
                       3 * (LEVEL - 1)
                      ) AS qstart,
           ADD_MONTHS (TRUNC (param.start_date, 'Q'), 3 * (LEVEL))
           - 1 AS qend
      FROM (SELECT TO_DATE ('&start_date') AS start_date,
                   TO_DATE ('&end_date') AS end_date
              FROM DUAL) param
CONNECT BY ADD_MONTHS (TRUNC (param.start_date, 'Q'), 3 * (LEVEL)) - 1 <=
                                                                param.end_date