Hello Larry,
The code works the same way it was working in certificate for Moodle 1.9 : 2 differents actions in the log table are considered as part of the same login session if they are separated by a time lesser that the session timeout value and time are only added between actions part of the same session. So In your case if the student does part of a course one day, shut his computer and log and complete the course next day the time of the first session would be from his first login to his latest action this day no more.
But you need to be aware of the way the calculation is done because if for instance session timeout is set to 1 hour on your Moodle website, and if a student login, do some action in the course, shut his computer, then login again 45 minutes later and do some action, he will be considered as having spend 45 minutes in course because all actions will be considered as part of the same session. Unfortunately I see no way to avoid that.