Cannot Group Dates in Pivot Tables

I have a pivot table with a list of dates. When I try to group them I get the "cannot group that selection" error.

There is no text in any of the cells excet for the very last one (Grand Total). I tried replacing "Grand Total" with a dummy date and got the same result.
The format of the dates is 2007/19/04. Is that the problem?

Did you ever figure out how to Summarizing Date Ranges within a Pivot Table.

I have received the same error message as you stated on your blog.