The start date will be when the task will become visible for students that have been assigned the task.
The due date is when students are supposed to complete the task by. This will be the date the task is displayed in the teacher planner.
The expiry date is when the task will no longer appear in the students' list of work to complete. Students can still work on the task after the due date if you set the expiry date to be great than 0.
For adaptive tasks: work completed in this time frame can be identified by a blue progress bar (instead of the usual green) on the results page for adaptive .
For custom tasks: the students will no longer see the task after it has expired.
If you want the students to work on the same task after the task has expired, you can duplicate the task and reassign to students.