Formula Help

I'm trying to display total sales from the previous month this is the formula i used:

PriorMonth:=CALCULATE(SUM(DATA[NonTaxableSalesAmt]),DATESMTD(DATA[InvoiceDate]))

But it just shows the sales from the current month

I am using excel 2013 powerpivot.

What am i doing wrong???

Is CALCULATE an Excel function?

Calculate is a DAX function

I didn't see this part

I am using excel 2013 powerpivot

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and I need another coffee