The format for calculating last month’s value is:
[Measure Name] = CALCULATE([Measure of Interest], PREVIOUSMONTH([Date Field in Date Dimension]))
Create the measure below in the OrderDetails table:
Total Including Tax Last Month = CALCULATE ( [Total Including Tax], PREVIOUSMONTH ( 'Date'[Date] ) )
We’re using the [Total Including Tax] measure within another measure. This is often considered best practice, as you have fewer calculations to maintain if you need to change the logic of your base measure.