Use GObject bind thing

src/applicationwindow.py

@@ -20,16 +20,12 @@ class ApplicationWindow(Gtk.ApplicationWindow):
 
         set_time_popover = TimePopover()
         self._set_time_button.props.popover = set_time_popover
-        set_time_popover.connect(
-            'notify::datetime', self._on_set_time_popover_notify_datetime
-        )
 
         self._clock = DigitalClock()
         self._main_box.add(self._clock)
         self._count = CountdownLabel()
         self._main_box.add(self._count)
 
-    def _on_set_time_popover_notify_datetime(
-        self, popover: GObject.Object, pspec: GObject.ParamSpec
-    ) -> None:
-        self._count.target = popover.props.datetime
+        set_time_popover.bind_property(
+            'datetime', self._count, 'target', GObject.BindingFlags.DEFAULT
+        )