systimestamp

Hi,
If my dbtimezone is set to +00:00 and my sessiontimezone is also +00:00 why systimestamp returns CST time zone? It should return greenich time instead?

SQL> ALTER SESSION SET time_zone='-00:00';

Session altered.

SQL> select systimestamp from dual;

SYSTIMESTAMP
---------------------------------------------------------------------------
12-MAR-12 03.48.29.684000 PM -05:00

SQL> SELECT dbtimezone,sessiontimezone FROM dual;

DBTIME
------
SESSIONTIMEZONE
---------------------------------------------------------------------------
+00:00
+00:00

Confused.

Thanks,
Sha