It's a little hard to believe that Discover does not allow you to do comparisons by date. In other words, the basic functionality to compare date ranges that exists in SiteCatalyst is not in Discover.
It seems like one sinple way to solve this would be to have date ranges at the report level so at least you could see, using the multi-report view, the difference between reports given different date ranges. Ideally, that would be an option when you run a comparision against two columns.
This ommission means I am once again exporting data to excel for actual analysis.