Datediff function or variations of it work fine generally but… if you want to have exact duration in Hours:Minutes or HH:MM format. You gotta do what you gotta do one of which is as follows.
This basically rounds off the hour and then uses mod function to calculate minutes.
=(DateDiff(DateInterval.Hour,Fields!StartTime.Value,Fields!EndTime.Value)) & ":" & ((DateDiff(DateInterval.Minute,Fields!StartTime.Value,Fields!EndTime.Value)) mod 60)
The code is written in SSRS expression format but can be converted into other formats easily and frankly if you cant then you are doing the wrong job.
Yeah I know its not exactly a huge deal but might help someone out there along with millions of solutions out there. So cheer up!