SQL> col dbtimezone      format a30
SQL> col sessiontimezone format a30
SQL> SELECT dbtimezone, sessiontimezone FROM dual;
DBTIMEZONE                     SESSIONTIMEZONE
------------------------------ ------------------------------
+00:00                         +02:00

SQL>
SQL> col tzabbrev format a5
SQL> SELECT UNIQUE tzname FROM v$timezone_names
  2   WHERE tzname in ('MST', 'PST', 'EST');
TZNAME
----------------------------------------------------------------
EST
MST
PST

SQL>
SQL> ALTER SESSION SET TIME_ZONE='PST';

Session altered.

SQL>
SQL> SELECT dbtimezone, sessiontimezone FROM dual;
DBTIMEZONE                     SESSIONTIMEZONE
------------------------------ ------------------------------
+00:00                         PST