If you wanted to include fields that show the date range along with the Sum, you can add those calculated fields to the query like this:
Code:
SELECT DateSerial(Year(Date()),IIf(Day(Date())>14,Month(Date())-1,Month(Date())-2),15) AS LowerDate, DateSerial(Year(Date()),IIf(Day(Date())>14,Month(Date()),Month(Date())-1),14) AS UpperDate, Sum(Transactions.Amount) AS SumOfAmount
FROM Transactions
WHERE (((Transactions.TransDate) Between DateSerial(Year(Date()),IIf(Day(Date())>14,Month(Date())-1,Month(Date())-2),15) And DateSerial(Year(Date()),IIf(Day(Date())>14,Month(Date()),Month(Date())-1),14))) OR (((Transactions.TransDate) Between DateSerial(Year(Date()),IIf(Day(Date())>14,Month(Date())-1,Month(Date())-2),15) And DateSerial(Year(Date()),IIf(Day(Date())>14,Month(Date()),Month(Date())-1),14)));