To find the difference between two datetimes

Hi all, How to we find the difference in hours between two datetimes using report builder? I have two date columns in dd/mm/yyyy hh:mm:ss format. If i have to find the difference in hours between two dates, how do i accomplish using report builder? I tried using datediff("HOUR", date1, date2) in the formula field , this gives me error,and is there any way to display the difference in hh:mm:ss format? Can anyone please help me with this?Thanks,s.Lekha