diff --git a/project/ui/app.slint b/project/ui/app.slint
index e3f19de..28d2a58 100644
--- a/project/ui/app.slint
+++ b/project/ui/app.slint
@@ -16,8 +16,8 @@ export component Prorater inherits Window {
     out property <string> last_billing: "NULL";
     out property <string> current_membership;
     out property <string> new_membership;
-    property <string> calculate_msg: "NULL";
-    callback on_calculate() -> string;
+    property <CalcResult> calculation_result;
+    callback on_calculate() -> CalcResult;
 
     VerticalBox {
         Image {
@@ -81,7 +81,7 @@ export component Prorater inherits Window {
             Button {
                 text: "Calculate";
                 clicked => {
-                    calculate_msg = on_calculate();
+                    calculation_result = on_calculate();
                     calculation.show();
                 }
             }
@@ -111,7 +111,9 @@ export component Prorater inherits Window {
         }
 
         calculation := Calculation {
-            message: calculate_msg;
+            message: calculation_result.message;
+            start: calculation_result.start;
+            end: calculation_result.end;
             x: (root.width - self.width) / 2;
             y: (root.height - self.height) / 2;
         }