Setting the MaxValue, MinValue and Value properties at runtime doesn't work when the settings are done in the Opened event handler of RadDatePicker. The DateTime values of the properties are updated as expected, but this is not reflected in the UI.

To work this around, you can use the Click event of the internal DateTimePickerButton, instead of the Opened event.

private void DatePicker_Loaded(object sender, RoutedEventArgs e)
{
	var btn = this.datePicker.FindChildByType<DateTimePickerButton>();
	btn.Click += Btn_Click;            
}

private void Btn_Click(object sender, RoutedEventArgs e)
{
	var vm = (MainViewModel)this.datePicker.DataContext;
	this.datePicker.MinValue = DateTime.Today;
}