Oracle 18c

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CREATE TABLE table_name ( high_date ) AS
SELECT '23-09-20' FROM DUAL UNION ALL
SELECT '15-AUG-99' FROM DUAL;
2 rows affected

ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD"T"HH24:MI:SS';
SELECT TO_DATE( high_date, 'DD-MM-RR' ) AS high_date
FROM   table_name;


HIGH_DATE


2020-09-23T00:00:00

1999-08-15T00:00:00