Even if they look like dates, some may really be text. You could try one
of the techniques shown here, to convert them to numbers:

http://www.contextures.com/xlDataEntry03.html

If that doesn't work, there are a few more suggestions he

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

Pascale wrote:
Hello
I am new at pivot table.
I am trying to group the dates contained in my column heading by month and I
always get the message "cannot group that selection"
I have checked that the data source used is properly formatted as a date.
Can someone help please? thanks


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