Number of hours worked calculation

Hello,

I am trying to set up a spreadsheet which does the following:

Start time Finish Time
09:00 17:00

if you use =HOUR(finish-start) it returns 8, but if you have this

Start time Finish Time
17:00 02:00

Then you get a #NUM! error.

Any ideas what formula you could key in to get it to calculate the number of hours worked in both cases?

Thanks

Simon