Fixed: SplitDateTime Widgets always complains about the date/time format.

src/events/models.py

@@ -112,7 +112,7 @@ class Event(models.Model):
             return "New Event Model"
 
     def clean(self):
-        if self.end < self.start:
+        if self.end and self.end < self.start:
             raise ValidationError({
                 'end': _("A event can't end before it had started")
             })