diff --git a/src/main.rs b/src/main.rs
index c7c50c9..2eace02 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,15 +1,25 @@
-// Based on example from https://crates.io/crates/gtk4
+// Based on examples
+// - https://crates.io/crates/gtk4
+// - https://github.com/gtk-rs/gtk4-rs/blob/main/examples/clock/main.rs
+// - https://github.com/gtk-rs/gtk4-rs/blob/main/examples/clipboard/main.rs
+// - https://github.com/kashifsoofi/blog-code-samples/blob/gtk4-c-counter-app/gtk4-rust-counter-app/src/main.rs
 
 use chrono::Local;
+use std::cell::Cell;
+use std::rc::Rc;
 
-use gtk4 as gtk;
+use gtk::glib;
+use gtk::glib::clone;
 use gtk::prelude::*;
-use gtk::{glib};
+use gtk4 as gtk;
+
+const APP_ID: &str = "de.privacy1st.pdt";
+
+const DUR_MIN: f64 = 1.0;
+const INTERVAL_SEC: u32 = 1;
 
 fn main() -> glib::ExitCode {
-    let application = gtk::Application::builder()
-        .application_id("de.privacy1st.pdt")
-        .build();
+    let application = gtk::Application::builder().application_id(APP_ID).build();
     application.connect_activate(build_ui);
     application.run()
 }
@@ -33,18 +43,28 @@ fn build_ui(app: &gtk::Application) {
         .spacing(24)
         .build();
 
+    // How to change variable using GTK4 Button? https://stackoverflow.com/a/76657798/6334421
+    let running = Rc::new(Cell::new(false));
+
     let button_start = gtk::Button::with_label("Start");
-    button_start.connect_clicked(|_| {
+    button_start.connect_clicked(clone!(@strong running => move |_| {
+        running.set(true);
         eprintln!("Timer started.");
-    });
+    }));
     container.append(&button_start);
 
     let button_stop = gtk::Button::with_label("Stop");
-    button_stop.connect_clicked(|_| {
-        eprintln!("Timer stopped.");
-    });
+    button_stop.connect_clicked(clone!(@strong running => move |_| {
+            running.set(false);
+            eprintln!("Timer stopped.");
+    }));
     container.append(&button_stop);
 
+    let mut progress: f64 = 0.0;
+    let progress_bar = gtk::ProgressBar::new();
+    progress_bar.set_fraction(progress);
+    container.append(&progress_bar);
+
     let time = current_time();
     let label = gtk::Label::default();
     label.set_text(&time);
@@ -56,14 +76,23 @@ fn build_ui(app: &gtk::Application) {
 
     // we are using a closure to capture the label (else we could also use a normal function)
     let tick = move || {
+        if running.get() {
+            progress += ((INTERVAL_SEC as f64) / 60.0) / DUR_MIN;
+            progress_bar.set_fraction(progress);
+        }
+
+        if progress > 1.0 {
+            // todo
+        }
+
         let time = current_time();
         label.set_text(&time);
         // we could return glib::ControlFlow::Break to stop our clock after this tick
         glib::ControlFlow::Continue
     };
 
-    // executes the closure once every second
-    glib::timeout_add_seconds_local(1, tick);
+    // executes the closure once every `INTERVAL_SEC` seconds
+    glib::timeout_add_seconds_local(INTERVAL_SEC, tick);
 }
 
 fn current_time() -> String {