conversion of high_value to date datatype
I think there is another solution to this.

DECLARE

   CURSOR c1 IS
   SELECT
      HIGH_VALUE
   FROM
      USER_TAB_PARTITIONS
   WHERE
      TABLE_NAME = 'MYTAB';

   ls_sql_statement   VARCHAR2(1000);
   ld_date_val        DATE;

BEGIN

   FOR c1rec IN c1 LOOP
      ls_sql_statement := 'SELECT '||c1rec.HIGH_VALUE||' FROM DUAL';
      EXECUTE IMMEDIATE ls_sql_statement INTO ld_date_val;
      --Do more stuff here with ld_date_val
   END LOOP;

END;
/