That error can occur if there are blank cells or cells with text in the
date column. There are some suggestions here for fixing the problem:

http://www.contextures.com/xlPivot07.html#Problems

Andy M wrote:
Hi All,

Although I am generally comfortable with Pivot tables I frequently run into
a problem grouping date ranges within the pivot table. Sometimes this works
fine and other times excel says that it cannot group that data. There is some
mention of this in the help however it does not suggest how to fix it. It
seems that it has to do with the date format from the original list however I
have tried reformatting and it seems to not help.

Has anyone ever figured this out? Some date ranges work fine and other
don't. Thanks for any help that you can offer.


--
Debra Dalgleish
Excel FAQ, Tips & Book List
http://www.contextures.com/tiptech.html