you do a datediff using seconds then convert that value into time.
if it never goes over 24 hours you can display it by adding the total seconds to any midnight value. If it goes over 24 hours then you will have to decide how you want it displayed (e.g. 25 hours or 1 day and 1 hour)and write a formula accordingly. Search this site as there are multiple postings with sample code for this exact request.