In the Performance Tab, when clicking on a specific assignment, could you add submission date/time to the list of visible columns?
Adding this as an option in "score display" could also work. Perhaps it could also be an option on the Gradebook screen, and not only the Activity Grader screen.
Hey Flew, with the new Activity Grader (must be enabled by an admin), you can now enable some submission details. For example, you can have the submission date appear in the table.
Check out today's release notes.