Christmas.cs

private DispatcherQueueTimer _timer;
private TextBlock _countdown;

public MainPage()
{
    this
        .Background(new ImageBrush().ImageSource("ms-appx:///ChristmasC/Assets/Background.jpg")
            .Stretch(Stretch.UniformToFill))
        .Content(_countdown = new TextBlock()
            .FontSize(48)
            .FontWeight(FontWeights.Bold)
            .TextAlignment(Microsoft.UI.Xaml.TextAlignment.Center)
            .VerticalAlignment(VerticalAlignment.Center)
            .HorizontalAlignment(HorizontalAlignment.Center)
            .Foreground(Colors.White));

    _timer = DispatcherQueue.CreateTimer();
    _timer.Tick += OnTick;
    _timer.Interval = TimeSpan.FromSeconds(0.5);
    _timer.Start();
}

private void OnTick(object? sender, object e)
{
    var christmasTime = new DateTime(DateTimeOffset.UtcNow.Year, 12, 25);
    var timeUntilChristmas = christmasTime - DateTime.UtcNow;

    if (timeUntilChristmas.TotalSeconds < 0)
    {
        _timer.Stop();
        _countdown.Text("Merry Christmas!");
        return;
    }
    else
    {
        _countdown.Text(
            $"{timeUntilChristmas.Days} days" + Environment.NewLine +
            $"{timeUntilChristmas.Hours} hours" + Environment.NewLine +
            $"{timeUntilChristmas.Minutes} minutes" + Environment.NewLine +
            $"{timeUntilChristmas.Seconds} seconds");
    }
}