When the submission deadline has passed and the submission is closed, no longer can documents or files be submitted in Canvas. If a teacher deems that there are valid reasons to accept a submission despite the delay, the teacher can now submit the student's document on behalf of the student in the course. This means that one no longer needs to extend the submission with a new date/time for the specific student anymore.
Here's what to do:
In Speedgrader, teachers can then see when it was submitted and by whom, and the student can see on their side when it was submitted and by whom.