About WinRT projection : the DateTime case (http://www.sandor.fr/en/2012/11/21/a-propos-de-la-projection-winrt-le-cas-datetime)

clean.cs

public DateTimeOffset? SelectedDate
{
    get { return (DateTimeOffset?)GetValue(SelectedDateProperty); }
    set { SetValue(SelectedDateProperty, value); }
}

public static readonly DependencyProperty SelectedDateProperty =
    DependencyProperty.Register("SelectedDate", typeof(DateTimeOffset?), typeof(DataTimePicker), new PropertyMetadata(null, OnSelectedDateChanged));

dirty.cs

public object SelectedDate
{
    get { return (DateTime?)GetValue(SelectedDateProperty); }
    set { SetValue(SelectedDateProperty, value); }
}

public static readonly DependencyProperty SelectedDateProperty =
    DependencyProperty.Register("SelectedDate", typeof(object), typeof(DataTimePicker), new PropertyMetadata(null, OnSelectedDateChanged));

dp.cs

public DateTime? SelectedDate
{
      get { return (DateTime?)GetValue(SelectedDateProperty); }
      set { SetValue(SelectedDateProperty, value); }
}

public static readonly DependencyProperty SelectedDateProperty =
      DependencyProperty.Register("SelectedDate", typeof(DateTime?), typeof(DataTimePicker), new PropertyMetadata(null, OnSelectedDateChanged));

private static void OnSelectedDateChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{

}

use.xaml

<local:DataTimePicker SelectedDate="{Binding CurrentDate}" />