Allowed null values in datetime field of appointment

smash/web/models.py

@@ -362,7 +362,8 @@ class Appointment(models.Model):
         verbose_name='Visit ID'
     )
     datetime_when = models.DateTimeField(
-        verbose_name='Appointment on'
+        verbose_name='Appointment on',
+        null=True
     )
     length = models.IntegerField(
         verbose_name='Appointment length (in minutes)'