Totalling hours from one day to the next

Hello, I have a problem that I have so far failed to find an answer to. My company works 24/7 and with that some people on nights starting prior to midnight and then finishing after. Of course the "regular" formula works well in calculating between 00:00 onwards but I am getting an error when for instance the person starts at 20:00 and finishes at 04:00. I have tried numerous ways but still the error persists, any ideas? Thank you.