DATE_PROMISED is still a string value.  It can't be grouped into periods, because strings don't come in periods (though periods do come in strings!   ).

If at all possible, I actually recommend doing the data conversion on the SQL side.  If you are able to alter the view/procedure you are using, add a field to convert DATE_PROMISED to an actual date.  If not, use the Add Command feature when building your data source to do so.

If that is not possible, you'll need to do the conversion in Crystal.  Create a formula, say @PromDate (because everyone needs one of those).  It should look like:

CDate({JC.10.DATE_PROMISED})

That's it.  Now, do your grouping on @PromDate.  It should work correctly now.