dynamic report level filter

I have a report that subscribes to a data feed. I do not have the options to create computed columns.

The data feed is financial aggregates. That is, each account has a value and a month that value is true for.

Spacely Sprockets, 101, January 2018

ACME, 123, January 2018

Spacely Sprockets, 94, December 2017

ACME, 131, December 2017

etc.

Using a matrix view, I need to show the values for each account this month and the same month last year.

I don't know how to do this without a computed column, and it appears that I can't use a measure in a report level filter.

For now I have put a slicer on the page and am relying on users to get it right. How do I automate this month selection?