You would have to use a custom report (such as RRDI) to add that kind of date/time break down.  As you said, there is only iteration in the OOTB/BIRT reports.  However, what about using the Browse features, you can add "tester" as a column, search on the modified, and exporting to Excel.  You could group by tester too.  Also, I've not explored what fields are in JRS, but if you use that and are on 5.0.2 or higher and it's there, that might be a possibility.  There is also a way in there to customized the SQL.