Dynamic Rolling Period Total vs. the Period a Year Earlier

[Anonymous]

Hi,

I am trying to compare sale's totals year over year for the rolling same months. For example, March & April of 2018 compared to March & April 2017, but also have this dynamically roll into the next month. I have the formula for the previous 2 months, but I don't a viable formula for other previous time period. 

This is what I have for the current year-

=Sum({<Date={"$(='>=' & Date(MonthStart(Max(Date), -2)) & '<' & Date(MonthStart(Max(Date))))"}>} LINE_EXT)

sum(LINE_EXT) is my sales total measure.

Any help is appreciated it,

Thanks