Previous Month Calculation Not working

Hey!

I am trying to get the previous date impressions and I am running into issues. I was able to calculate current month without a date table, but when I got to previous month I needed to add one, When I did and created a relationship between the two tables the data got messed up and was wrong.  I am essentially trying to do current month - previous month here is my formula for previous month, any help would be greatly appreciated!