Currently, when Delete is pressed the min values is set, however, the textbox should be cleared. 

This can be handled by using a custom provider:
public class MyFreeFormDateTimeProvider : FreeFormDateTimeProvider
{
    public MyFreeFormDateTimeProvider(string mask, CultureInfo culture, RadMaskedEditBoxElement owner)
        : base(mask, culture, owner)
    { }
    public override void KeyDown(object sender, KeyEventArgs e)
    {
        if (e.KeyValue != 46)
        {
            base.KeyDown(sender, e);
        }         
    }
}