Rather easy …
- Create two "One line of text" columns named Archive and Extract.
- Optionally add a validation formula to ensure that users enter an appropriate time string.
Add a computed column with a formula similar to the following:
= TEXT (TIMEVALUE ( [Check Out] ) – VALUE OF TIME ( [Check In] ), "hh: mm: ss")
Depending on the type of analysis you need, you can add an additional calculated column that just returns the numeric value. Set the return type to Number and you will get a fraction of a day. (1 = 24 hours, .5 = 12 hours, etc.)
= TIMEVALUE ( [Check Out] ) - VALUE OF TIME ( [Check In] )
Or to get the result in hours:
= (TIMEVALUE ( [Check Out] ) - VALUE OF TIME ( [Check In] )) * 24