Hi Florian,
User Config\Administrative Templates\Start Menu and Taskbar\Remove Clock
from the system notification area
At least Microsoft Windows XP Professional or Windows Server 2003 family
"Prevents the clock in the system notification area from being displayed.
If you enable this setting, the clock will not be displayed in the system
notification area. If you disable or do not configure this setting, the
default behavior of the clock appearing in the notification area will
occur."
The relevant registry setting is
HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer!HideClock
br,
Denis