DECLARE
@FirstDate datetime,
@LastDate datetime
SELECT
@FirstDate = '2000-01-01 09:00:00',
@LastDate = '2000-01-01 11:30:00'
SELECT CONVERT(varchar(12),
DATEADD(minute, DATEDIFF(minute, @FirstDate, @LastDate), 0), 114)
/* Results: 02:30:00:000 */
/* For less precision */
SELECT CONVERT(varchar(5),
DATEADD(minute, DATEDIFF(minute, @FirstDate, @LastDate), 0), 114)
/* Results: 02:30 */