diff --git a/project/src/app.rs b/project/src/app.rs
index 3a586c2..3be477f 100644
--- a/project/src/app.rs
+++ b/project/src/app.rs
@@ -1,11 +1,10 @@
 // Libraries
 slint::include_modules!();
+use super::Backend;
 use slint::{ModelRc, SharedString, VecModel};
 use std::io::Result;
 use std::rc::Rc;
 
-use super::Backend;
-
 // Macros
 macro_rules! map_err {
     ($expr:expr) => {
@@ -62,16 +61,51 @@ impl App {
     pub fn start(&self) -> Result<()> {
         // Creating an Rc of our App
         let app = Rc::clone(&self.app);
+        let backend = Rc::clone(&self.backend);
 
         // Setting the on_submit function
         self.app.on_on_calculate(move || {
             // Getting the prices of the stuff
             let current_membership = app.get_current_membership();
             let new_membership = app.get_new_membership();
+            let last_billing = app.get_last_billing();
 
-            // DEBUG
-            println!("Current Membership: {}", current_membership);
-            println!("New Membership: {}", new_membership);
+            // Are either of these a "Select One..."
+            if current_membership == "Select One..."
+                || new_membership == "Select One..."
+                || last_billing == "NULL"
+            {
+                return SharedString::from(
+                    "Please fill out all fields before calculating the Prorate.",
+                );
+            }
+
+            // Making the Backend Calculate This
+            let calculation = backend
+                .calculate(
+                    current_membership.into(),
+                    new_membership.into(),
+                    last_billing.into(),
+                )
+                .unwrap();
+
+            // Is there no cost difference?
+            if calculation.invalid {
+                return SharedString::from("No price difference, don't change anything.");
+            }
+
+            // Returning the Result
+            if calculation.reversed {
+                SharedString::from(format!(
+                    "Retract the billing date to {} (-{}).",
+                    calculation.new_date, calculation.days_pushed
+                ))
+            } else {
+                SharedString::from(format!(
+                    "Extend the billing date to {} (+{}).",
+                    calculation.new_date, calculation.days_pushed
+                ))
+            }
         });
 
         // Starting our application