Crystal Reports date formatted as string, how to group

Hi Experts -
I have a report showing open orders, both past due and future - the ship date (a formula named @due_date) being Group#1.  I needed order lines past due to to display 'Past Due', rather than the date, and the future shipments to show the ship date.   I accomplished this by converting all the dates to text, doing a comparison, and using the text where appropriate.  However, now I've lost the ability to group my dates by month - since they are not dates anymore.  Now I have a six page report with a group header for each ship date, instead of the desired summary showing all shipments by month.
Any ideas how to get the grouping back?  Or did I handle the group formula incorrectly?

Original formula, returns error of 'String Required..' at last line :
if {@Due Date} < currentdate() then
'Past Due '
else
{@Due Date}

So, I did this:
if totext({@Due Date}, "MM/dd/yyyy") < totext(currentdate(), "MM/dd/yyyy") then
'Past Due as of '
else
totext({@Due Date}, "MM/dd/yyyy")

Works, but as mentioned, I can't group text fields by month.

Thanks!