try this :
=IF( AND ( DaysDifference<=7 ,DaysDifference>=0) ,"7",IF(DaysDifference<=15,"15",IF(DaysDifference<=30,"30",IF(DaysDifference<0,"Overdue"))))
=IF( AND ( DaysDifference<=7 ,DaysDifference>=0) ,"7",IF(DaysDifference<=15,"15",IF(DaysDifference<=30,"30",IF(DaysDifference<0,"Overdue"))))