Use the start date for the due date is the end date is nil

app/models/customer_appointment.rb

@@ -30,7 +30,8 @@ class CustomerAppointment < ActiveRecord::Base
   ]
 
   def due_date
-    end_date&.to_date
+    return end_date&.to_date if end_date.present?
+    return start_date&.to_date
   end
 
   def self.visible(user)