public class DatePickerHelper

    {

        public static object GetWatermark(DependencyObject obj)

        {

            return (object)obj.GetValue(WatermarkProperty);

        }

        public static void SetWatermark(DependencyObject obj, object value)

        {

            obj.SetValue(WatermarkProperty, value);

        }

        // Using a DependencyProperty as the backing store for Watermark.  This enables animation, styling, binding, etc...

        public static readonly DependencyProperty WatermarkProperty =

            DependencyProperty.RegisterAttached("Watermark", typeof(object), typeof(DatePickerHelper),

                new FrameworkPropertyMetadata(null,

                    new PropertyChangedCallback(OnWatermarkChanged)));

        private static void OnWatermarkChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)

        {

            var datePicker = d as DatePicker;

            if (datePicker == null)

                return;

            if (datePicker.IsLoaded)

            {

                SetWatermarkInternal(datePicker, e.NewValue);

            }

            else

            {

                RoutedEventHandler loadedHandler = null;

                loadedHandler = delegate

                {

                    datePicker.Loaded -= loadedHandler;

                    SetWatermarkInternal(datePicker, e.NewValue);

                };

                datePicker.Loaded += loadedHandler;

            }

        }

        private static void SetWatermarkInternal(DatePicker d, object value)

        {

            var textBox = d.Template.FindName("PART_TextBox", d) as Control;

            if (textBox != null)

            {

                var watermarkControl = textBox.Template.FindName("PART_Watermark", textBox) as ContentControl;

                if (watermarkControl != null)

                    watermarkControl.Content = value;

            }

        }

    }