Difference betweeen two cummulative values

Hi, Anonymous , Anonymous 

First of all, thanks for the quick responses. It have been useful. 

Anonymous. I tried phormula you said and it shows an error. I think it's because Cummulative Quantity is a measure in the model, instead of a colum.

Anonymous. The phormula you said works, except for the phormula to calculate 'Cumul Qty'. 

So, I have changed phormula por Cumul Qty and it works.

Cummulative Quantity = 

CALCULATE(SUM(Sales[Quantity]);FILTER(ALL('Calendar'); 'Calendar'[Date] <= MAXX('Calendar';'Calendar'[Date])))

Diff = 
	[Cummulative Quantity]
	- CALCULATE(
		[Cummulative Quantity];
		FIRSTDATE( 'Calendar'[Date] )
	)

And it works.....

Thanks