WPF Countdown Timer

MainWindow.xaml

<Window x:Class="StoryboardDemo.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <TextBlock x:Name="CountdownDisplay" FontFamily="Calibri" FontSize="60"/>
    </Grid>
</Window>

MainWindow.xaml.cs

using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Media.Animation;

namespace StoryboardDemo
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            MouseDoubleClick += (o, args) => StartCountdown(CountdownDisplay);
        }

        private void StartCountdown(FrameworkElement target)
        {
            var countdownAnimation = new StringAnimationUsingKeyFrames();

            for (var i = 5; i > 0; i--)
            {
                var keyTime = TimeSpan.FromSeconds(5 - i);
                var frame = new DiscreteStringKeyFrame(i.ToString(), KeyTime.FromTimeSpan(keyTime));
                countdownAnimation.KeyFrames.Add(frame);
            }
            countdownAnimation.KeyFrames.Add(new DiscreteStringKeyFrame(" ", KeyTime.FromTimeSpan(TimeSpan.FromSeconds(6))));
            Storyboard.SetTargetName(countdownAnimation, target.Name);
            Storyboard.SetTargetProperty(countdownAnimation, new PropertyPath(TextBlock.TextProperty));

            var countdownStoryboard = new Storyboard();
            countdownStoryboard.Children.Add(countdownAnimation);
            countdownStoryboard.Completed += CountdownTimer_Completed;
            countdownStoryboard.Begin(this);
        }

        private void CountdownTimer_Completed(object sender, EventArgs e)
        {
            MessageBox.Show("Time's up!");
        }
    }
}