Subtract previous month values from current month values using DAX

Hi,

I suggest having a proper calendar dimension table.

I tried to create a sample pbix file like below.

You can try to create table by using below DAX formula after clicking "New Table" button.

And, create a relationship like below.

Please check the attached pbix file.

OFFSET function (DAX) - DAX | Microsoft Learn

Sales vs. previous sales: = 
VAR _currentmonth =
    SUM ( sales[sales] )
VAR _previousmonth =
    CALCULATE (
        SUM ( sales[sales] ),
        OFFSET (
            -1,
            ALL ( 'calendar'[Month-Year], 'calendar'[Month-Year sort] ),
            ORDERBY ( 'calendar'[Month-Year sort], ASC )
        )
    )
RETURN
    _currentmonth - _previousmonth