To change date format in silverlight

just add this code app.xaml.cs file

Add Name space are :
using System.Globalization;
using System.Threading;



public partial class App : Application
{
        private void Application_Startup(object sender, StartupEventArgs e)
        {
            CultureInfo ci = new CultureInfo(Thread.CurrentThread.CurrentCulture.Name);
            ci.DateTimeFormat.ShortDatePattern = "DD/MM/YYYY";
            Thread.CurrentThread.CurrentCulture = ci;
        }
}