Create 3 measures as shown below, and then add those 3 measures in the report along with a month slicer as shown below. You can change the month in the slicer and verify that the measure values change for the selected month.
Sales (Selected Month) = SUM ( Sales[Sales] )
Sales Last Year = CALCULATE ( SUM ( Sales[Sales] ), SAMEPERIODLASTYEAR ( ‘Date'[Date] ) )
Sales YTD = TOTALYTD ( SUM ( Sales[Sales] ), ‘Date'[Date] )
This is the first time I’ve seen a What If parameter in use. Very interesting.