Index: src/Products.Archetypes/Products/Archetypes/Field.py
===================================================================
--- src/Products.Archetypes/Products/Archetypes/Field.py	(revision 12299)
+++ src/Products.Archetypes/Products/Archetypes/Field.py	(working copy)
@@ -1449,6 +1449,9 @@
                 # which is assumed to be the local one
                 # see http://dev.plone.org/plone/ticket/10141
-                value +=  ' ' + DateTime().timezone()
+                original = value
                 value = DateTime(value)
+                local = DateTime()
+                if value.tzoffset() == 0 and local.tzoffset() != 0:
+                    value = DateTime(original + ' ' + local.timezone())
             except DateTime.DateTimeError:
                 value = None