public class CustomTimpePicker : RadTimePicker
{
    public override string ThemeClassName
    {
        get
        {
            return typeof(RadTimePicker).FullName;
        }
    }

    protected override RadTimePickerElement CreateTimePickerElement()
    {
        return new CustomRadTimePickerElement();
    }
}

public class CustomRadTimePickerElement : RadTimePickerElement
{
    protected override Type ThemeEffectiveType
    {
        get
        {
            return typeof(RadTimePickerElement);
        }
    }
    public override object Value
    {
        get
        {

            return base.Value;

        }

        set
        {
            if (value != null && value != DBNull.Value)
            {
                FieldInfo fi = typeof(RadTimePickerElement).GetField("editBox", BindingFlags.Instance | BindingFlags.NonPublic);
                RadMaskedEditBoxElement editBox = (RadMaskedEditBoxElement)fi.GetValue(this);
                PropertyInfo info = editBox.GetType().GetProperty("MaxDate", BindingFlags.Instance | BindingFlags.NonPublic);
                info.SetValue(editBox, new DateTime(this.MaxValue.Year, this.MaxValue.Month, this.MaxValue.Day, this.MaxValue.Hour, this.MaxValue.Minute, this.MaxValue.Second), null);
            }
        }
    }
}