diff --git a/stoptime.rb b/stoptime.rb
index b82af71..fba20c3 100644
--- a/stoptime.rb
+++ b/stoptime.rb
@@ -411,6 +411,18 @@ module StopTime::Models
         subtotal + vattotal
       end
     end
+
+    # Returns if the invoice is past due (i.e. it has not been paid within
+    # the required amount of days).
+    def past_due?
+      not paid? and (Time.now - created_at) > 30.days # FIXME: hardcoded!
+    end
+
+    # Returns if the invoice is past due (i.e. it has not been paid within
+    # the required amount of days).
+    def way_past_due?
+      past_due? and (Time.now - created_at) > 2 * 30.days
+    end
   end
 
   # == The company information class