Here is one example:
DECLARE @Date1 DATETIME, @Date2 DATETIME
SET @Date1 = '2008-05-09 15:59'
SET @Date2 = '2008-05-11 16:32'
SELECT convert(varchar(5),DATEDIFF(minute, @Date1, @Date2)/60) + ':' + convert(varchar(2),datediff(minute, @Date1, @Date2)%60)
Hope this will help you to...