public RadForm1()
 {
     InitializeComponent();

     radDateTimePicker1.DateTimePickerElement.TextBoxElement.MaskType = MaskType.FreeFormDateTime;

     this.radDateTimePicker1.DateTimePickerElement.TextBoxElement.Provider = new CustomMaskDateTimeProvider(this.radDateTimePicker1.DateTimePickerElement.TextBoxElement.Mask, this.radDateTimePicker1.Culture, this.radDateTimePicker1.DateTimePickerElement.TextBoxElement);

 }
public class CustomMaskDateTimeProvider : MaskDateTimeProvider
{
    public CustomMaskDateTimeProvider(string mask, CultureInfo culture, RadMaskedEditBoxElement owner) : base(mask, culture, owner)
    {
    }

    public override void ResetCurrentPartValue(object sender, KeyEventArgs e)
    {
        if (this.List == null)
        {
            return;
        }
        base.ResetCurrentPartValue(sender, e);
    }
}