Correct - MDX.  Oops, perhaps I posted this on the wrong forum.

For anyone else, found the solution to my issue.

member [Measures].[NumCustomers] as
        count    (
                    filter    (
                                 exists([Customer].[Customer Key].[Customer Key].members , {[Customer_Purchase_List]})
                                , (
                                     [Date].[Date].[All].[AggRangeOther]

For anyone else who has a similar issue - you need to aggregate the field.  https://stackoverflow.com/questions/37047150/how-to-query-date-field-in-mdx-with-and-without-aggregate-group