Power Pivot :: Creating Measure That Looks Only At Previous X Number Of Days

I have a table with date in the following format:

orderID | dateOrdered | customerID 

101 | 01/01/2015 | 1
102 | 02/02/2015 | 2
103 | 05/06/2015 | 3 

I need to create a measure that tells me how many customer's placed an order in the previous 91 days of any given date, not necessarily today. 

So, if you imagine a PivotTable which has a series of dates down the left hand side and 'totalNoCustomersLast60Days' as the only column, I need each day to tell me how many customers ordered in the x number of days.

For example if we say 60 days my table would look like the following:

Each row in this table would be telling me that "x number of customers placed orders within the period 60 days prior to,and including, the date on the left".

I have tried numerous ways of doing this and despite thinking it should be incredible simple it eludes me.