datetime : provide in UI all timezones supported by the DateTime library

also commit the new values on the fields "date" in proxy field libraries as
they are not automatically recomputed for now when changing config on a proxy
field

product/Formulator/Widget.py

@@ -2,7 +2,7 @@
 import string
 from DummyField import fields
 from DocumentTemplate.DT_Util import html_quote
-from DateTime import DateTime
+from DateTime import DateTime, Timezones
 from cgi import escape
 import types
 from DocumentTemplate.ustr import ustr
@@ -1280,9 +1280,7 @@ class MultiCheckBoxWidget(MultiItemsWidget):
 
 MultiCheckBoxWidgetInstance = MultiCheckBoxWidget()
 
-gmt_timezones =  [('GMT%s' %zone, 'GMT%s' %zone,) for zone in range(-12, 0)]\
-                  + [('GMT', 'GMT',),] \
-                  + [('GMT+%s' %zone, 'GMT+%s' %zone,) for zone in range(1, 13)]
+gmt_timezones = [(x, x) for x in sorted(set(Timezones()))]
 
 class DateTimeWidget(Widget):
   """