In order to get the required behavior in the DatePicker control you will need to customize the CalendarStyle and create a custom MonthButtonStyleSelector. We prepared and attached a sample project for you representing the described approach. Notice that the non required months are not removed, but disabled. There is no way to completely remove them as the MonthButtons are put in an UniformGrid in code-behind and this behavior cannot be overridden.
Hope this is helpful to achieve the needed behavior.
the Telerik team