Difference between 2 columns in a Matrix Table

Hi all,

I was wondering if someone could help me.

I want to calculate the difference between two columns on a Matrix Table

At row Level I have Department.

At Column Level I have Date. This is an actual Date column within the Data

At Values Level I have headcount. This is a measure and the following formula has been used: DISTINCTCOUNT(‘Employee’[Person ID])

I essentially want to compare the difference in Headcount between two specific dates.

For Example:

April                       May                       Difference

Headcount          Headcount          Headcount

10                           9                              -1

I have read other blog post and I have tried the following formulas

Previous Month = CALCULATE([HEADCOUNT],FILTER(ALL(‘Employee’[Date].[Month]),MAX(‘Employee’[Date])-1))

Difference = IF(ISBLANK([Previous Month]=0),CALCULATE([Headcount]-[Previous Month]))

By the looks of the results I am getting Mays Data.

When using the difference formula I am getting blank spaces

Can anyone tells me were in going wrong with this?

Thanks