DATESBETWEEN Dynamic Monthly

I've exhausted myself failing to solve what should almost literally be the simplest DAX measure of my career. I simply want a measure that sums values for the period 15 months to 12 months prior to whatever month is selected by users on the slicer/slider. DATESBETWEEN examples I've found only show hardcoded dates, which is useless--this needs to shift dynamically. DATEADD fails due to "contiguous dates" issues.

The simple logic is:  

mTotalSalesBetween15Mo&12MoAgo:=CALCULATE([mTotalSales], DATESBETWEEN(-15,-12, MONTH))

Does anyone have any insight? I can't go through another "Finkle and Einhorn" night over something that should be so dead simple.