VAR _ThisMonth = CALCULATE ( SUM ( 'Products (2)'[Amount] ) )
VAR _PreviousMonth = CALCULATE ( SUM ( 'Products (2)'[Amount] ), PREVIOUSMONTH ( 'Products (2)'[Date] ) )
_ThisMonth - _PreviousMonth
Thanks to the great efforts by MS engineers to simplify syntax of DAX! Most beginners are SUCCESSFULLY MISLED to think that they could easily master DAX; but it turns out that the intricacy of the most frequently used RANKX() is still way beyond their comprehension!