it is pretty simple to add or remove time zones.
Proceed like the following:
1. Open the Zope Administration page in your browser.
I'll assume that it is reachable as localhost:8080, you might have to change the computer name.
2. You now see a kind of directory showing folders and plain objects.
Please select the folder Custom here
3. You are now in the custom folder and you'll find a python script called timeZones
(It'll probably be the last entry in the list).
Open this script (i.e. click on it and Zope will offer you an editor page)
4. You now see a long, very long list of all time zones we could find.
Somewhere there you'll find the required time zone.
Remove the hash sign and save it.
Now the Pacific time zone will be available to the GUI.
After removing references to Europe/Berlin (our time zone), you can repeat the above steps and comment it out, if desired.