I've just finished working out how to run vbscript from within an SD Procedure
So if I get a chance tomorrow I'll see if I can create one that takes the start and end date/times and works out the difference and then writes the difference to a 3rd Custom Field.
So once it is computed and available in the custom field you can run reports etc from it.
So just a matter of do I store the calculated result value as minutes or days . I vote for minutes
Cheers
Paul