erp5_production_planning: set date only

avoid to trigger interaction when edit for the first time

bt5/erp5_production_planning/SkinTemplateItem/portal_skins/erp5_production_planning/ProductionPlanning_init.py

@@ -13,7 +13,7 @@ else:
   stop_date = addToDate(start_date, {'month':1})
 
 context.edit(
-  start_date=start_date,
-  stop_date=stop_date,
+  start_date = start_date.Date(),
+  stop_date = stop_date.Date(),
   title = context.Base_translateString('Production Planning at ${date}', mapping= {'date': start_date.Date()}),
   production_planning_period = context.portal_preferences.getPreferredProductionPlanningPeriod())