How to calculate duration between two date/time fields?

I have two date/time fields (both will always be in the past) from which I need to calculate a duration. (i.e. the difference between one and the other.) Ideally, I could display it as hours:minutes, but I'll settle for just hours if I have to.

I tried this:

abs(subtract(number($time_out_c),number($time_in_c)))

but it yields nothing. (a blank field)

I'm a complete new to using calculated fields. Any help would be appreciated.