https://stackoverflow.com/questions/4975098/wpf//-textbox-use-default-text

            TextBox tb = sender as TextBox;
            if (e.Changes.Count == 1)
                {
                if (e.Changes.ElementAt(0).AddedLength == 1 && (tb.Text.Length == 2 ))
                    {
                    tb.Text += ":";
                    tb.SelectionStart = tb.Text.Length;