The formula provided by Blake doesn't seem to work for me. For past dates it returns due in xx days and for future dates, it returns overdue. Also, it will only return 15 days overdue, when it could actually be 30, 60 90+.
I created this, which seems to work and provides 'Due in xx days', 'Overdue xx days' and 'Due Today'.
=IF(ISBLANK(O10),"",IF(DAYS(TODAY(),O10)<0,CONCATENATE("Due in ",-DAYS(TODAY(),O10)," Days"),IF(DAYS(TODAY(),O10)>0,CONCATENATE("Overdue ",DAYS(TODAY(),O10)," Days"),"Due Today")))