You're welcome, rubella.
On my RH8 box, I click with right mouse button on the date/time in the Gnome panel.
Then I select "adjust date & time" from the context menu that appears.
After supplying the root password, I can change the date, time, whether or not NTP is to be used as well as the timezone.
seems to bring up the same configuration GUI.
Maybe there's some graphical config for that in your menu too. Personnally, I haven't worked with Debian yet, so I can't provide more practical help.