To reproduce:
 radTimePicker1.Value = DateTime.Now.AddDays(10);
 radTimePicker1.TimePickerElement.MaskedEditBox.MaskType = Telerik.WinControls.UI.MaskType.FreeFormDateTime;

Workaround:

protected override void OnLoad(EventArgs e)
{
    base.OnLoad(e);
    radTimePicker1.Value = DateTime.Now.AddDays(10);

    var provider = radTimePicker1.TimePickerElement.MaskedEditBox.Provider as FreeFormDateTimeProvider;

    provider.MinDate = DateTime.MinValue;
    provider.MaxDate = DateTime.MaxValue;
}