Once a task or quiz is open, learners can submit it at any point during the course. All submissions are flagged with a time-stamp, which makes it visible to facilitators exactly when that task was submitted.
When a task has passed it's due date, learners see it as overdue.
To ease administrative loads for facilitators, we still technically allow learners to submit after the due date and time. That means it's up to you (and/or your organisation) whether you "accept" that submission or not.
If an entire class needs an extension, you do have the option to change the due date for an assessed task for the entire class (an iQualify manage user can do this by editing the class activation).
Other articles you may find useful
How to view and mark tasks - Facilitators and coaches can view automarked task details and provide marks and feedback on manually marked tasks.
The Class Console - Class Console is built especially for facilitators and replaces the Class and Marking pages for tasks courses.
Understanding task statuses - Understand the different statuses for assessed and non-assessed tasks.
Reset and Reopening submissions - As a Facilitator you can reopen or reset a learners submission, but what is the difference?