Variation Column

Please see the below if it works for you.... 

2 Steps:  

Step 1 Count of related rows as a column in Date Table:

QP Row Count = IF(COUNTROWS(RELATEDTABLE(QP))>0,COUNTROWS(RELATEDTABLE(QP)), 0)

Step 2: Difference in Count from Prior Month:

Difference From Prior Month = 'Date Table'[QP Row Count] - CALCULATE(SUM('Date Table'[QP Row Count]),DATEADD('Date Table'[Date],-1,MONTH))

Data ModelQP Table

Step 1 - Add a Related Row Count ColumnStep 2 - Add a Difference in Row Count from prior Month