Oracle 21c

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ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
SELECT (TO_DATE('153312', 'HH24MISS') - TO_DATE('000000', 'HH24MISS')) DAY(0) TO SECOND(0)
        AS time_column
FROM   DUAL;

TIME_COLUMN

+0 15:33:12

SELECT TO_TIMESTAMP('153312', 'HH24MISS') - TO_TIMESTAMP('000000', 'HH24MISS')
        AS time_column
FROM   DUAL;

TIME_COLUMN

+000000000 15:33:12.000000000