Sql For Financial Reporting Periods This Month, Last Month, This Quarter, Last Quarter, This Year, Last Year

Does anyone know of a way to use a funtion for returning records based on fiscal reporting periods like Quickbooks uses for example "This Month", "Last Month", "This Quarter", "Last Quarter", "This Year", "Last Year". While I realize that I can create a very long date time parsing routine  for this but it is not very elegant or useful. I thought there might be a way to do this already with an existing function.
I have created a stored procedure that I pass a @ViewRange Parameter to and it returns the records that I want but I need this ability in several procedures and wanted to turn it into a stored procedure.
IF @ViewRange = 'This Month'
SELECT TOP 20 Customer.LastName AS Customer, SUM(Sales.AmtCharge) AS Amount FROM Customer INNER JOIN Sales ON Customer.CustNo = Sales.CustNo WHERE (MONTH(Sales.InvDate) = MONTH(CURRENT_TIMESTAMP)) AND (YEAR(Sales.InvDate) = YEAR(CURRENT_TIMESTAMP)) GROUP BY Customer.LastName ORDER BY SUM(Sales.AmtCharge) DESC;
IF @ViewRange = 'Last Month'
SELECT TOP 20 Customer.LastName AS Customer, Sum(Sales.AmtCharge) AS Amount FROM Customer INNER JOIN Sales ON Customer.CustNo = Sales.CustNo WHERE(MONTH(Sales.InvDate) = MONTH(CURRENT_TIMESTAMP) - 1) And (YEAR(Sales.InvDate) = YEAR(CURRENT_TIMESTAMP)) GROUP BY Customer.LastName ORDER BY Sum(Sales.AmtCharge) DESC;
 Any ideas?