Intersection - Retention Rate

Here's the measure:

// Assumptions:
// 1. There's a dimension that stores Customers.
// 2. There's a fact table, Sales, that joins on
// 	  CustomerId to Customers and Date to Calendar.
// 3. There's a dimension Calendar which is a proper
//	  calendar in the model (marked as such).

[Retention Rate] =
var __lastVisibleDate = LASTDATE( 'Calendar'[Date] )
var __shouldCalculate =
	// You can calculate the ratio only when
	// you can go back in time 4 months from
	// __lastVisibleDate.
	NOT(
		ISBLANK( NEXTDAY( DATEADD(__lastVisibleDate, -4, month ) ) )
		&&
		ISBLANK( DATEADD( NEXTDAY( __lastVisibleDate ), -4, month ) )
	)
var __result =
	if( __shouldCalculate, 
	
		var __custWithPurchWithinLast3Months =
			CALCULATETABLE(
				VALUES( Sales[CustomerID] ),
				DATESINPERIOD(
					'Calendar'[Date],
					__lastVisibleDate,
					-3,
					MONTH
				)
			)
		var __lastVisibleDateMinus3Months =
			dateadd( __lastVisibleDate, -3, month )
		var __custWithPurch1MonthBeforeLast3Months =
			CALCULATETABLE(
				VALUES( Sales[CustomerID] ),
				DATESINPERIOD(
					'Calendar'[Date],
					__lastVisibleDateMinus3Months,
					-1,
					MONTH
				)
			)
		var __custThatBoughtInBothPeriods =
			INTERSECT(
				__custWithPurchWithinLast3Months,
				__custWithPurch1MonthBeforeLast3Months
			)
		var __ratio =
			DIVIDE(
				countrows( __custThatBoughtInBothPeriods ),
				COUNTROWS( __custWithPurchWithinLast3Months )
			)			
		return
			__ratio
	)
RETURN
	__result

Please bear in mind that this works for ANY SELECTED PERIOD OF TIME, not only for months.

Best

D