Change group options using a formula?

My users have asked me for a report that would display grouped data in two different formats: by month or by week.  The catch is that they want to be able to choose between the two options at runtime.  I currently have the data grouped by date, then by project number, with the detail records hidden and available in a drill-down.  They would want the top-level data to look something like the example below.

July 2008        $100,000
     Project1          $30,000
     Project2          $50,000
     Project3          $20,000

-----or-----

Week 29          $45,000
     Project1          $10,000
     Project2          $15,000
     Project3          $20,000

My first approach was going to be to create a parameter the users could set, then change the group display option to print either by week or by month depending on that value.  But I can't find anywhere to put the formula!

Am I missing something obvious?  Is there a better way to approach the problem?  I really don't want to create two separate versions of the same report over a formatting difference.