Problem with extending DateTimeEditBox and using the "MMM" custom format specifier

Hi,

I have extended the DateTimeEditBox class with functionality for auto-selecting all the text when the user clicks a "Part" or uses the left and right arrow keys: 

public class MyDateTimeEditBox : DateTimeEditBox
{
	public MyDateTimeEditBox()
	{
		PreviewMouseDown += new MouseButtonEventHandler(MyDateTimeEditBox_PreviewMouseDown);
		PreviewKeyDown += new KeyEventHandler(MyDateTimeEditBox_PreviewKeyDown);
	}

	void MyDateTimeEditBox_PreviewMouseDown(object sender, MouseButtonEventArgs e)
	{
		Part part = e.Source as Part;
		if (part != null)
		{
			MaskedTextBox mtb = (MaskedTextBox)VisualTreeHelperExtended.GetFirstDescendant(part, typeof(MaskedTextBox));
			if (mtb != null)
			{
				mtb.Focus();
				mtb.SelectAll();
				e.Handled = true;
			}
		}
	}

	void MyDateTimeEditBox_PreviewKeyDown(object sender, KeyEventArgs e)
	{
		MaskedTextBox mtb = Keyboard.FocusedElement as MaskedTextBox;
		if (mtb != null)
		{
			if (e.Key == Key.Right)
			{
				mtb.MoveFocus(new TraversalRequest(FocusNavigationDirection.Right));
				mtb = Keyboard.FocusedElement as MaskedTextBox;
				mtb.SelectAll();
				e.Handled = true;
			}
			else if (e.Key == Key.Left)
			{
				mtb.MoveFocus(new TraversalRequest(FocusNavigationDirection.Left));
				mtb = Keyboard.FocusedElement as MaskedTextBox;
				mtb.SelectAll();
				e.Handled = true;
			}
		}
	}
}

This works fine except when I'm using the "MMM" custom format specifier. When I set Format="dd MMM yyyy HH:mm:ss", I'm not able to click the month-part, and when I use the left and right arrow-keys, the month-part is skipped. When tracing the code, I can see that it enters the "MyDateTimeEditBox_PreviewMouseDown" with the correct MaskedTextBox, but the month-part will not become selected.

If I remove the e.Handled=true in "MyDateTimeEditBox_PreviewMouseDown", I can click the month part, but the text is not selected.

Is this a "missing feature" or do I need to implement it in another way?

Thanks,

Geir