Incorrect Measure Total - subtracting between two Month over Month measures

[hyggins]

Hello,

I'm trying to get my Total Changes measure to calculate correctly. I'm using a matrix visual. Red is what I get currently, Green is the desired total.

Year	Month	This Month	Last Month	Monthly Variance
2019	Jan	19,223,321	18,290,500	932,821
2019	Feb	19,651,173	19,223,321	427,852
2019	Mar	19,891,796	19,651,173	240,623
			Current Total	40,475,790
			Desired Total	1,601,296

Measures:
This Month = SUM(Table[Widgets])
Last Month = CALCULATE([This Month], PREVIOUSMONTH(DimDate[Date]))
Monthly Variance = [This Month] - [Last Month]

Final Measure = IF(HASONEVALUE(DimDate[Year]) && HASONEVALUE(DimDate[Month])
                        ,[Monthly Variance]
                        ,?? // this line is what I'm trying to solve

                       )

I've been scouring the forums all day and can't seem to get this to work. I've tried variations using SUMX and SUMMARIZE but have yet to be successful.

Any assistance is very much appreciated! Thank you!

Grant

[parry2k · Accepted solution]

@hyggins try to change your measure like below and then it should work and total of your monthly variance will be correct.

Last Month = 
CALCULATE(
[This Month],
DATEADD( DimDate[Date], -1, MONTH )
)

[hyggins]

@parry2k Wow! That did it! Thank you so much!

I'm going to have to dig into the nuances of PREVIOUSMONTH vs DATEADD to see what the differences are. 

Thanks again!

Grant