Adding a "costumized" total to report

Hello Together,

I built a report in ms ssrs based on a cube.

My task is creating a overwiev for revenues of the last two years, the column groups are grouped by months of the year. After creating such a matrix I added a total to the report.

The Problem now is that I want to add a total for the months, meaning: a total wich begins with January until the last ended month ( in this case September).

I´m working with this expression: "=DatePart("m",Now())-1" to get  to get September.

I also inserted a new column group and added this expression into the header and set a filter with this expression.

Is this in general in Reporting Services possible or do I have to make this in the Data Set?

Any ideas

Thanks

JO_MO