diff --git a/addons/calendar/calendar.py b/addons/calendar/calendar.py
index 410e83661df..3d9ab8b3d3f 100644
--- a/addons/calendar/calendar.py
+++ b/addons/calendar/calendar.py
@@ -851,10 +851,11 @@ class calendar_event(osv.Model):
         if values.get('start_datetime') or values.get('start_date') or values.get('start') \
                 or values.get('stop_datetime') or values.get('stop_date') or values.get('stop'):
             allday = values.get("allday", None)
+            event = self.browse(cr, uid, id, context=context)
 
             if allday is None:
                 if id:
-                    allday = self.read(cr, uid, [id], ['allday'], context=context)[0].get('allday')
+                    allday = event.allday
                 else:
                     allday = False
                     _logger.warning("Calendar - All day is not specified, arbitrarily set to False")
@@ -871,10 +872,16 @@ class calendar_event(osv.Model):
                         values[fld] = values['%s_%s' % (fld, key)]
 
             diff = False
-            if allday and values.get('stop_date') and values.get('start_date'):
-                diff = datetime.strptime(values['stop_date'].split(' ')[0], DEFAULT_SERVER_DATE_FORMAT) - datetime.strptime(values['start_date'].split(' ')[0], DEFAULT_SERVER_DATE_FORMAT)
-            elif values.get('stop_datetime') and values.get('start_datetime'):
-                diff = datetime.strptime(values['stop_datetime'].split('.')[0], DEFAULT_SERVER_DATETIME_FORMAT) - datetime.strptime(values['start_datetime'].split('.')[0], DEFAULT_SERVER_DATETIME_FORMAT)
+            if allday and (values.get('stop_date') or values.get('start_date')):
+                stop_date = values.get('stop_date') or event.stop_date
+                start_date = values.get('start_date') or event.start_date
+                if stop_date and start_date:
+                    diff = datetime.strptime(stop_date.split(' ')[0], DEFAULT_SERVER_DATE_FORMAT) - datetime.strptime(start_date.split(' ')[0], DEFAULT_SERVER_DATE_FORMAT)
+            elif values.get('stop_datetime') or values.get('start_datetime'):
+                stop_datetime = values.get('stop_datetime') or event.stop_datetime
+                start_datetime = values.get('start_datetime') or event.start_datetime
+                if stop_datetime and start_datetime:
+                    diff = datetime.strptime(stop_datetime.split('.')[0], DEFAULT_SERVER_DATETIME_FORMAT) - datetime.strptime(start_datetime.split('.')[0], DEFAULT_SERVER_DATETIME_FORMAT)
             if diff:
                 duration = float(diff.days) * 24 + (float(diff.seconds) / 3600)
                 values['duration'] = round(duration, 2)