DAX

Hey,

do you have a separate Calendar table, if not you should ;-) And make sure both tables are related properly.

Then you should give PREVIOUSMONTH a try like so:

CALCULATE(SUM('yourtable'[yournumericcolumn]), PREVIOUSMONTH('calendar'[Date]))

Here you will find almost everything about time intelligence with DAX:

http://www.daxpatterns.com/time-patterns/

If you are new to the sometimes mind-boggling DAX statements I would wholeheartedly recommend the following books

 also in this sequence

1. Analyzing Data with Microsoft Power BI and Power Pivot for Excel
2. Dax Patterns 2015
3. The Definitive Guide to DAX

All three have been written by Marco Russo and Alberto Ferrari

Hope this gets you started 

The subtraction can look like this

CALCULATE(SUM('yourtable'[yournumericcolumn])) - CALCULATE(SUM('yourtable'[yournumericcolumn],PREVIOUSMONTH('calendartable'[date]))

Cheers