It is really really easy to change the default NLS_DATE_FORMAT setting but to be honest, you should set it at a session level IMHO.
We basically just need 
to run “ALTER SYSTEM SET NLS_DATE_FORMAT=’YYYY-MM-DD’ SCOPE=SPFILE” as a user with sysdba privileges. If you started the Oracle instance without a spfile (it should be located at $ORACLE_HOME/dbs/spfile[instance name].ora), you will receive the ORA-32001 error.
SQL> ALTER SYSTEM SET NLS_DATE_FORMAT=‘YYYY-MM-DD’ SCOPE=SPFILE;
ALTER SYSTEM SET NLS_DATE_FORMAT=‘YYYY-MM-DD’ SCOPE=SPFILE
*
ERROR at line 1:
ORA-32001: WRITE TO SPFILE requested but no SPFILE specified at startup
ALTER SYSTEM SET NLS_DATE_FORMAT=‘YYYY-MM-DD’ SCOPE=SPFILE
*
ERROR at line 1:
ORA-32001: WRITE TO SPFILE requested but no SPFILE specified at startup
Just create a new spfile, restart:
SQL> SELECT INSTANCE_NAME FROM v$instance;
INSTANCE_NAME
—————-
UAT2
SQL> CREATE spfile=‘/oracle/10g/dbs/spfileUAT2.ora’ FROM pfile=‘/oracle/10g/dbs/initUAT2.ora’;
*restart*
SQL> SELECT INSTANCE_NAME FROM v$instance;
INSTANCE_NAME
—————-
UAT2
SQL> ALTER SYSTEM SET NLS_DATE_FORMAT=‘YYYY-MM-DD’ SCOPE=SPFILE;
System altered.
*restart*
SQL> SELECT value FROM v$nls_parameters WHERE parameter =‘NLS_DATE_FORMAT’;
VALUE
—————————————————————-
YYYY-MM-DD
That’s it. 
