Assistance with Calculation

SumHeadcount = VAR CurrentMonth LASTDATE('YourTable'[Date])
VAR LastMonthNum = if(Month(CurrentMonth) = 1, 12, MONTH(CurrentMonth) - 1)
VAR LastMonth = if(Month(CurrentMonth) = 1, DATE(YEAR(CurrentMonth]) - 1, LastMonthNum, 1), DATE(YEAR(Date]), LastMonthNum, 1))
RETURN
SUM('YourTable'[ValueField]) - CALCULATE(
	SUM('YourTable'[ValueField]),
	ALL('YourTable')
	'YourTable'[Date] = LastMonth
)

This might do it, just put it on something with Month Context.  This assumes your 'date' fields are done in a way that each date aligns to a single date within a month, like the first.

I wrote something like this for another post.  Not the only way to do this, but it was the first way that came to mind:

SumHeadcount = VAR CurrentMonth LASTDATE('YourTable'[Date])
VAR LastMonthNum = if(Month(CurrentMonth) = 1, 12, MONTH(CurrentMonth) - 1)
VAR LastMonth = if(Month(CurrentMonth) = 1, DATE(YEAR(CurrentMonth]) - 1, LastMonthNum, 1), DATE(YEAR(Date]), LastMonthNum, 1))
RETURN

CALCULATE(
	SUM('YourTable'[ValueField]),
	ALL('YourTable')
	'YourTable'[Date] >= LastMonth,
	'YourTable'[Date] <= CurrentMonth
)

 EDIT:  The code above doesnt do exactly your thing, but it might give you something to work from

Hi Kolumam,

Have you resolved your issue? If you have, please mark the right/helpful reply as answer. More people will new things here. If you haven't, please feel free to ask.

Best Regards,
Angelia