you can use this in sql server 2005
here is an eg
create table timediff (time1 varchar(50),time2 varchar(50));
insert into timediff values('2011-12-09 08:40:00.000','2011-12-09 09:00:00.000');
SELECT convert(varchar,CAST(time2 AS DATETIME)-CAST(time1 AS DATETIME), 8)TIME_DIFF from timediff;