Calculating Hour Outside Core Hours When All Hours Fall Outside Core

I am trying to set up a time sheet for staff where it calculates hours worked in and outside of core hours. I can do a calculation to work out what they have worked in and out of core hours providing that part of their time starts of finishes in core times. The problem I am having is when some one works only outside core hours. Our core hours are 6am to 6pm. The problem is when they work from say 7pm to 1am. This is irregular work paid as overtime not shift work.