Measure to show customer census

[GiantPygmy]

I'm attempting to create a measure to show historical numbers for current customers. Current customers is defined as a customer who has had an order in the 3 months prior or 3 months after selected month. I have a fact table (F_Orders) with the order date, customer ID, and Order number. I also have date table and customer table with customers actual information on it that it joins to. I want to be able to create a pivot table with the month as the column and the year as a slicer that will show monthly current customers based on the definition above.