Settings of NLS_DATE_FORMAT parameters for the session are not taken into account in query results (QUERY RESULT). However, they work correctly in script results (SCRIPT OUTPUT) or in SQLcl.
select sysdate from dual;
QUERY RESULT :
(interesting fact: if I copy this date (right click I got '2024-02-18T17:37:36' )
SCRIPT RESULT
SYSDATE
--------
18/02/24
alter session set nls_date_format = 'yyyy-mm-dd hh24:mi:ss';
select sysdate from dual;
QUERY RESULT
When I copy it (right click ) => ‘2024-02-18T17:39:59’
SCRIPT RESULT
SYSDATE
-------------------
2024-02-18 17:40:41
In SQL Developer and sqlcl - it works correctly.
SQL> select sysdate from dual;
SYSDATE
___________
24/02/18
SQL> alter session set nls_date_format = 'yyyy-mm-dd hh24:mi:ss';
Session altered.
SQL> select sysdate from dual;
SYSDATE
______________________
2024-02-18 17:25:55
SQL>