Is it possible that some of your dates are actually text that look like dates?
Save the file.
Then, in a blank cell type =ISNUMBER(A1), where A1 represents one of your
dates. If it returns FALSE, it is actually text and not a date. You can pull
that formula down to test all of the dates.
If your values are text, you can Group Manually by selecting all the dates
you want to Group and choosing Data | Group and Outline | Group.
Alternatively you'll need to convert the text to dates.
You would likely want to do this in the data source, if possible. The method
of this may vary slightly depending on where the data is stored.

tj

" wrote:

I am trying to group a column of dates by year and month, and I've done
it before and it's worked fine. But now it's saying "Cannot group on
that Selection". I checked the data and it's in date format and there
aren't any blank or null values. It seems like this is a bug because
I've done it before with the same exact data. Have any of you had this
problem? What am I doing wrong?

Thanks!

Lis