Test steps:
1. As a teacher in a course, create an assignment.
2. Set the Due Date and Cut off date to be five minutes from your current time, leaving other settings as default.
3. Save the assignment and wait 5 minutes.
4. Log in as a student and verify you do not see the submission button and cannot submit.
5. Log back in as a teacher; navigate to the assignment and click "View all submissions"
6. For the student in question, click the icon in the Edit column and select "Grant extension"
7. Check the box to enable extensions and select a date in the PAST
8. Verify that you get an error message explaining extensions must be after the due date.
9. Now select a date in the future.
10. Verify in the gradebook that the student's Status shows an extension has been granted.
11. Log back in as the student and verify you can now see the submission button and can now submit.