SQL & PL/SQL

We are using Oracle 12c.

I used the following in Oracle SQL developer.

alter session set nls_date_format = 'mm/dd/yy hh24:mi:ss';

select  WHENCREATED from mytable;

It did not change the format of the date, it is still like the 07-NOV-15 08.00.20.000000000 AM. The server default one.

I also tried on SQLPLUS, same thing.

I know there is a way to set in preference the date format, but just wonder why the alter session set nls_date_format does not work for my existing date column format in this session?

If I just query

select sysdate from dual; that works.

Thanks,