select partition_name from user_tab_partitions 
where table_name = ... and to_date(substr(high_value, ...), 'yyyy-mm-dd') < (sysdate - :p_interval));

with
  tmp_xml as 
  (
    select 
        dbms_xmlgen.getxmltype('select table_name, partition_name, partition_position, high_value from user_tab_partitions where table_name = ''MY_TABLE''') as x
    from dual
  ),
  tmp_list as
  (
    select 
        extractValue(rws.object_value, '/ROW/PARTITION_NAME')     partition_name
      , extractValue(rws.object_value, '/ROW/PARTITION_POSITION') partition_position
      , extractValue(rws.object_value, '/ROW/HIGH_VALUE')         high_value
    from tmp_xml x, table(xmlsequence(extract(x.x, '/ROWSET/ROW'))) rws
  )
  select
      partition_name
  from tmp_list
  where to_date(substr(high_value, 11, 10), 'yyyy-mm-dd') < sysdate - :p_interval;

DECLARE
  v_obj_id                all_objects.object_id%TYPE;
  v_archive_depth         NUMBER := 30;
  e_partition_not_exists  EXCEPTION;
  PRAGMA exception_init(e_partition_not_exists, -02149);

BEGIN
    SELECT
      data_object_id
    INTO v_obj_id
    FROM all_objects o, all_tab_partitions p
    WHERE p.table_owner = 'MY_SCHEMA' 
      AND p.table_name = 'MY_TABLE'
      AND p.partition_position = 1
      AND o.owner = p.table_owner
      AND o.object_name = p.table_name
      AND o.subobject_name = p.partition_name;

    FOR cpart IN (
                  SELECT * FROM(
                                WITH T AS (SELECT TRUNC(log_date) d
                                          FROM my_table PARTITION (dataobj_to_partition(my_table, v_obj_id))
                                          WHERE ROWNUM = 1)
                                SELECT D+ROWNUM-1 part_date FROM t
                                  CONNECT BY LEVEL < TRUNC(sysdate) - (SELECT D FROM T)
                              )
                  WHERE part_date < TRUNC(sysdate) - v_archive_depth
                  )
    LOOP
        BEGIN
            EXECUTE IMMEDIATE 'ALTER TABLE my_table DROP PARTITION FOR (TO_DATE('''||to_char(cpart.part_date, 'yyyymmdd') || ''',''YYYYMMDD''))';
        EXCEPTION
            WHEN e_partition_not_exists THEN NULL;
        END;
      END LOOP;
EXCEPTION
  WHEN no_data_found THEN NULL;
END;