1. At creation time
SQL> CREATE DATABASE ...
SET TIME_ZONE='Europe/London';
If not specified with the CREATE DATABASE statement, the database time zone
defaults to the server’s O/S timezone offset.

2. After database creation, use the ALTER DATABASE SET TIME_ZONE statement and
then shut down and restart the database.
SQL> ALTER DATABASE SET TIME_ZONE = '-05:00';
SQL> ALTER DATABASE SET TIME_ZONE = 'Europe/Lisbon';
The change will not take effect until the database is bounced.

select dbtimezone from dual;
-04:00
select sessiontimezone from dual;
+03:00.


select new_time(sysdate,to_char(current_timestamp,'TZD'),'GMT') from dual; -- this is for US/Easter Time Stamp