NLS_DATE_FORMAT is a parameter heavily dependent on a session. A good practice is to write a logon trigger depending on the application requirement to set this parameter write at the session logon. It can be done via a small script for a logon trigger.
[sourcecode language="sql"] CREATE OR REPLACE TRIGGER SCOTT.CHANGE_DATE_FORMAT AFTER LOGON ON DATABASE WHEN ( USER='SCOTT' ) begin execute immediate 'alter session set nls_date_format = ''DD/MM/YYYY HH24:MI:SS'' '; end ; / [/sourcecode]