Even though a "due date" is set, when students are "required to click the submit button", they can do so after the deadline. This causes their submitted file to display as "late"; teachers cannot tell if the actual submission was on time (and the "submit button click" was after the deadline), or if the student actually missed the deadline.

This may seem like a "minor issue", but for our large courses and mostly not perfectly "Moodle-aware" student graders, this is a large issue: it often seems as "if the student managed to cheat and submit late". For privacy reasons, the graders cannot access the log to verify if this was actually the case.

Steps to reproduce under all Moodle versions since 2.3, including 2.4.* and 2.5.*:
1. Create a new assignment, enable "due date" and set it to a date, and set "Prevent late submissions" to "Yes". The actual type of the submission (online text / file submission) does not matter. Set the option "Require students click submit button" to "Yes".
2. As a student, submit a solution before the due date, but do NOT click on the "Submit" before the due date.
3. As a student, wait until the deadline is past; the form will display a message "Assignment is overdue by: ...". Note that the "Submit assignment" button remains visible.
4. As a student, press the "Submit assignment" button and confirm your choice by clicking "Continue". Your assignment is now labelled as "late".
5. As a teacher, access the student submission for grading. Even the file that was uploaded on time is now flagged as "late".

I also propose a quick fix for this problem. This causes the "submit button" to disappear and thus also keeps the time stamp of the file valid. Additionally, it does NOT set the student submission to "submitted for grading", which means that if the deadline is set to a later date, the student can still edit his or her solution, instead of seemingly having pressed the "submit" button.

In mod/assign/locallib.php, method "view_student_summary($user, $showlinks), change line 2089 from

$showsubmit = $submission && ($submission->status == ASSIGN_SUBMISSION_STATUS_DRAFT) && $showlinks;

to

$showsubmit = $submission && ($submission->status == ASSIGN_SUBMISSION_STATUS_DRAFT) && $showlinks && $this->submissions_open();

That is, include the information whether the submission is still "open" into the determination whether the submit button should be shown or not.