ALTER FUNCTION dbo.checkMaxInv
	(
	@starting Datetime,
	@ending   Datetime
	
	)
RETURNS    @CustMostRecent TABLE (CustCode varchar (12) primary key,
								   CustName varchar (30),
								   SalesID  varchar (12),
								   InvDate  Datetime) 
AS
	BEGIN
		 INSERT INTO @CustMostRecent
		   SELECT Billing.CustCode, CustCode.CustName, CustCode.SalesID, MAX(Billing.InvDate) AS mostR
FROM Billing INNER JOIN CustCode ON Billing.CustCode = CustCode.CustCode 
WHERE (Billing.InvDate BETWEEN @starting AND @ending) AND (Billing.InvoiceTotal > 0) 
GROUP BY Billing.CustCode, CustCode.CustName, CustCode.SalesID
	RETURN
	END

CREATE TABLE #Billing 
    (CUSTCODE VARCHAR
    ,SALESID VARCHAR
    ,AMTPAIDSOFAR INT
    ,INVDATE DATETIME);

CREATE TABLE #CUSTCODE 
    (CUSTCODE VARCHAR
    ,CUSTNAME VARCHAR(20)
    ,SALESID VARCHAR);

INSERT INTO #CUSTCODE
SELECT '1', 'Dwain', '1' UNION ALL SELECT '2', 'llcooldre', '2' UNION ALL SELECT '3', 'Dr Dobbs', '3'

INSERT INTO #Billing
SELECT '1', '1', 400, '2012-01-01'
UNION ALL SELECT '1', '2', 400, '2012-01-20'
UNION ALL SELECT '2', '1', 400, '2012-01-31'
UNION ALL SELECT '2', '2', 400, '2012-02-28'
UNION ALL SELECT '2', '3', 400, '2013-01-01'
UNION ALL SELECT '3', '1', 400, '2011-01-01'
UNION ALL SELECT '3', '2', 400, '2011-02-01'
UNION ALL SELECT '3', '3', 400, '2011-03-01'

DECLARE @StartDate DATE = '2012-12-01'
    ,@EndDate DATE = '2012-12-31'

;WITH EligibleInvoices AS (
    SELECT CUSTCODE, SALESID, AMTPAIDSOFAR, INVDATE
        ,MostRecent=ROW_NUMBER() OVER (PARTITION BY CUSTCODE ORDER BY INVDATE DESC)
    FROM #Billing) 
SELECT a.CUSTCODE, a.CUSTNAME
    ,MostRecent=CASE WHEN DATEADD(day, 365, INVDATE) > @StartDate THEN INVDATE ELSE NULL END
FROM #CUSTCODE a
LEFT JOIN EligibleInvoices b ON a.CUSTCODE = b.CUSTCODE
WHERE MostRecent = 1 AND AMTPAIDSOFAR > 0

DROP TABLE #Billing
DROP TABLE #CUSTCODE

... DateColumn >= @ StartDate and DateColumn < @EndDate

declare @StartDate datetime = '20120301', -- Using datetime as the data is date/time oriented
           @EndDate datetime = '20120401'; -- or dateadd(mm,1,@StartDate)
... DateColumn >= @ StartDate and DateColumn < @EndDate

... DateColumn >= @ StartDate and DateColumn < @EndDate

declare @StartDate datetime = '20120301', -- Using datetime as the data is date/time oriented
           @EndDate datetime = '20120401'; -- or dateadd(mm,1,@StartDate)
... DateColumn >= @ StartDate and DateColumn < @EndDate

DECLARE @StartDate DATE = '20120301',
             @EndDate DATE = '20120331';

... DateColumn BETWEEN @StartDate AND @EndDate


... DateColumn >= @StartDate and DateColumn <= @EndDate