I would either create a public function in a module of business calculations
or a table of day ranges. I generally consider nesting more than 2 IIf()s a
mistake. The following is the function method. You can search google groups
for a table of ranges.

Public Function GetAging(datStart As Date, datEnd As Date) As String
   Dim intDays As Integer
   intDays = DateDiff("d", datStart, datEnd)
   Select Case intDays
       Case Is <= 14
           GetAging = "0-14 Days"
       Case Is <= 30
           GetAging = "15-30 Days"
       Case Is <= 60
           GetAging = "31-60 Days"
       Case Is <= 90
           GetAging = "61-90 Days"
       Case Else
           GetAging = "90 Days and Over"
   End Select
End Function

PARAMETERS Forms!frmDate.dtpFrom DateTime;
TRANSFORM Sum(Debtors.AmountOS) AS TotalOS
SELECT Debtors.Acc, Debtors.AccName, Debtors.InvNo, Debtors.Note
FROM Debtors
GROUP BY Debtors.Acc, Debtors.AccName, Debtors.InvNo, Debtors.Note
PIVOT GetAging([tDate],forms!frmDate.dtpFrom) In ("0-14 Days","15-30
Days",..etc..);