>>

>>SELECT SUM(CAST(DATETIME1 - DATETIME2 AS int))  AS NUMDAYS
>>

>>>
>
>Thank you, Sergey. I wonder if anyone ever tested what is faster, DATEDIFF() or CAST().

DATEDIFF would be faster.

Try Boris's sample:

DECLARE @Test TABLE (Fld1 datetime, Fld2 datetime)
DECLARE @i int
SET @i = 0
WHILE @i < 1000
      BEGIN 
         INSERT INTO @Test VALUES (GETDATE()+@i, GETDATE()+@i*20)
         SET @i = @i +1
      END
set statistics time on  
    
SELECT SUM(cast(Fld2-Fld1 as int)) FROM @Test
SELECT SUM(DATEDIFF(dd,Fld1,Fld2)) FROM @Test