declare @TestDate datetime;
select datediff(dd,getdate(),@TestDate)
select datediff(dd,@TestDate,getdate())
set @TestDate = '20130318';
select datediff(dd,getdate(),@TestDate)
select datediff(dd,@TestDate,getdate())

-----------
NULL

(1 row(s) affected)


-----------
NULL

(1 row(s) affected)


-----------
11

(1 row(s) affected)


-----------
-11

(1 row(s) affected)