Oracle 21c

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ALTER SESSION SET NLS_DATE_FORMAT = 'DD-MM-RR';
SELECT TO_CHAR(CAST('01-01-07' AS DATE), 'YYYY-MM-DD HH24:MI:SS') FROM DUAL;

TO_CHAR(CAST('01-01-07'ASDATE),'YYYY-MM-DDHH24:MI:SS')

2007-01-01 00:00:00

ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD';
SELECT TO_CHAR(CAST('01-01-07' AS DATE), 'YYYY-MM-DD HH24:MI:SS') FROM DUAL;

TO_CHAR(CAST('01-01-07'ASDATE),'YYYY-MM-DDHH24:MI:SS')

0001-01-07 00:00:00

ALTER SESSION SET NLS_DATE_FORMAT = 'DD-MON-RR';
SELECT TO_CHAR(CAST('01-01-07' AS DATE), 'YYYY-MM-DD HH24:MI:SS') FROM DUAL;
ORA-01843: not a valid month