diff --git a/project/ui/app.slint b/project/ui/app.slint
index acb11a3..712fec1 100644
--- a/project/ui/app.slint
+++ b/project/ui/app.slint
@@ -6,9 +6,10 @@ export component Prorater inherits Window {
     height: 720px;
 
     in property <[string]> membership_names: ["Select One..."];
+    out property <string> last_billing: "NULL";
     out property <string> current_membership;
     out property <string> new_membership;
-    callback on_calculate();
+    callback on_calculate() -> string;
 
     VerticalBox {
         Image {
@@ -72,9 +73,15 @@ export component Prorater inherits Window {
             Button {
                 text: "Calculate";
                 clicked => {
-                    on_calculate();
+                    result_text.text = on_calculate();
                 }
             }
+
+            result_text := Text {
+                text: "";
+                font-size: 1.25rem;
+                horizontal-alignment: center;
+            }
         }
 
         date_picker := DatePickerPopup {
@@ -84,7 +91,8 @@ export component Prorater inherits Window {
 
             accepted(date) => {
                 date_picker.close();
-                last-billing-date.text = date.month + "/" + date.day + "/" + date.year;
+                root.last_billing = date.month + "/" + date.day + "/" + date.year;
+                last_billing_date.text = root.last_billing;
             }
 
             canceled => {