Group Field in Excel 2007 was disabled

Problem:

The Group Field button in MS Excel 2007 was disabled when I selected a date column in my Pivot Table. At first glance, my data looked good, and I could not figure out why the button would be disabled. I added other columns that contained date information and the button would become enabled for them.

Solution:

At a closer inspection of my spreadsheet data, I had an invalid date in my source table. It was hardly noticeable as it was an additional slash in the date, but it was enough for the pivot wizard to not recognize the column as a date column. I fixed this bad date and all was well.

Note:

I'm logging this here because I spent about 45 minutes searching around for "Group Field button disabled" and the like and could not find anything. While this was entirely my fault, it would have been nice to have a hint to inspect my data closer.