I tried the ISNUMBER formula and they all came back as TRUE, so that's
not it. I know for sure that they are stored as dates, so that's why
this is so weird to me. Thanks anyway - I appreciate your response.
:-)

tjtjjtjt wrote:
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