Use of SUM and COUNT with GROUP BY

Erland,

This still gives me "14" months for "Child" (should be "7"). Below's the code that I have. For the periods taht you specified, I'll need the result to be 2, since the member was active in Feb as well as March.

IF OBJECT_ID('tempdb..#Test') IS NOT NULL DROP TABLE #Test
GO

DECLARE @LastDayOfPrevYear DATE = (SELECT CONVERT(DATE,DATEADD(yy, DATEDIFF(yy,0,getdate()) + 1, -1)))
DECLARE @LastDayOfPrevMonth DATE = (SELECT CONVERT(DATE,DATEADD(MONTH, DATEDIFF(MONTH, -1, GETDATE())-1, -1)))

CREATE TABLE #Test
(
        [State] CHAR(2),
        DOB DATE,
        StartDt DATE,
        EndDt DATE,
        [Desc] VARCHAR(8),
        Category CHAR(1),
        MemberID VARCHAR(10),
        SubID VARCHAR(2),
        Code VARCHAR(5)
)

INSERT INTO #Test
VALUES
('NC','1/30/2013','1/1/2014','1/31/2014','Child','B','123456','2','38'),
('NC','1/30/2013','1/1/2014','1/31/2014','Child','B','123456','2','39'),
('NC','1/30/2013','2/1/2014','5/31/2014','Child','B','123456','2','38'),
('NC','1/30/2013','2/1/2014','5/31/2014','Child','B','123456','2','39'),
('NC','1/30/2013','6/1/2014','7/31/2014','Child','B','123456','2','38'),
('NC','1/30/2013','6/1/2014','7/31/2014','Child','B','123456','2','39'),
('NC','1/17/2014','1/1/2014','7/31/2014','Infant','S','456789','1','49'),
('NC','2/4/2014','2/1/2014','7/31/2014','Infant','S','246376','3','49')

SELECT 
          [State]
        , [Desc]
        , Category
        , COUNT(DISTINCT str(MemberID) + str(SubId) + str(Code)) AS TotalCnt
        , SUM(DATEDIFF(MONTH, StartDt, DATEADD(DAY, 1, EndDt))) AS Months
        , SUM
                (DATEDIFF(MONTH, StartDt,
                CASE 
                WHEN EndDt >= @LastDayOfPrevYear
                THEN @LastDayOfPrevMonth
                ELSE EndDt
                END) + 1) AS CalMonths -- This is what I have
FROM   #Test
GROUP BY 
          [State]
        , [Desc]
        , Category