如何报告TFS 2008中某一周的已完成工作？

We're on Team Foundation Server 2008 and I'm trying to find a way to report on the change in completed work from week to week at the task level. The MDX query below works pretty well, but I'd like to get rid of need to hard code last week's date. I've tried using prevmember and parallelperiod without success, but I'm no MDX expert. We're on Team Foundation Server 2008 and I'm tr