Set cron timezone with default value

Closes #12056

trytond/trytond/ir/cron.py

@@ -102,9 +102,13 @@ class Cron(DeactivableMixin, ModelSQL, ModelView):
         # Migration from 5.0: remove required on next_call
         table_h.not_null_action('next_call', 'remove')
 
-    def get_timezone(self, name):
+    @classmethod
+    def default_timezone(cls):
         return tz.SERVER.tzname(datetime.datetime.now())
 
+    def get_timezone(self, name):
+        return self.default_timezone()
+
     @classmethod
     def check_xml_record(cls, crons, values):
         pass