If a learner has already self-enrolled in a course/learning item before you assign it to them, you can add a due date to the learner's enrolment from the course Manage page.
Select Self-directed from the left navigation, followed by the status of the learner's progress against the course: Not Started, In Progress, or Completed.
From self-directed, select the checkbox at the left of each learner, then an action bar will appear at the bottom of the page with the button: Add assignment.
After that, you will see a pop up window, containing the ability for you to specify a due date for the selected learners > Select the due date > Then click on "Update"
The selected learners will receive an email notification, informing them of the date by which they need to complete that learning. If the learner's completed learning is outside of your current compliance period, you will then need to reassign the learner to the course or resource.