Change DatePickerTextBox value

Question

Hello I changed the value of the DatePicketTextBox :

<DatePicker Grid.Column="3"
            Grid.Row="1"
            SelectedDate="{Binding Order.DateIn, Mode=TwoWay}"
            Width="200"
            Height="30"
            HorizontalAlignment="Left"
            VerticalContentAlignment="Center"
            Margin="10 0 0 20">
    <DatePicker.Resources>
        <Style TargetType="DatePickerTextBox">
            <Setter Property="Text" Value="Selecteer een datum" />
        </Style>
    </DatePicker.Resources>
</DatePicker>

When I start the application and click on "New Order" it shows this (the correct values) :

If I then click on another product where for example the "Besteldatum" is set :

And then click on the button "New Order" it shows the English default value again for the field "Besteldatum". And it seems that it shows again the default value when the value was set. Hope it is clear, any ideas?

EDIT :

Here is a better overview, the list with orders based on year and at the far righ the "Nieuwe bestelling" ("New Order") button :

Here is the code to show the "Nieuwe betselling" UserControl :

private void OrderUserControl(Order order)
{
    ShowOrderUserControl = Visibility.Visible;

    Products = _productService.GetProducts();

    if (order == null)
    {
        Order = new Order();

        OrderUserControlTitle = "Nieuwe bestelling toevoegen";
    }
    else
    {
        Order = order;

        SelectedProduct = Products.Find(f => f.Id == Order.ProductId);

        OrderUserControlTitle = "Bestelling aanpassen";
    }
}