CountDown/MainWindow.xaml

<Window x:Class="CountDown.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:CountDown"
        mc:Ignorable="d"
        Title="Bientôt la retraite !!!" Height="180" Width="180"
        Background="Transparent"
        WindowStyle="ToolWindow"
        ShowInTaskbar="false">
    <Grid>
        <TextBlock Text="{Binding Counter,StringFormat={}{0:dd\\jhh\\hmm\\mss\\'}}" HorizontalAlignment="Center" VerticalAlignment="Bottom" FontSize="28" Foreground="White" FontFamily="Agency FB"/>
    </Grid>
</Window>