Traditional project management systems put expected completion dates on each task.  Interestingly, systems to support agile development, such as Trac our our internal task and ticket systems, do not put dates on tasks.
Why is it bad to put due dates on tasks?  It creates a lot unproductive work for the project manager.  When you actually do the work, you almost always do things in a different order than you expect.  So, if you want to make your plan accurate, you have to edit the due dates.  Changing one due date changes all the other due dates that are after the task you are editing.  Editing due dates, over and over again, is a waste of time.

The difference between agile project management and traditional project management is that agile admits the reality - priorities change, and dates change.  So, no dates at the task level.

 It is much easier to assign a priority to the ticket, and then work on the tickets in priority order.  If you change the priority on one ticket, or if takes longer than you expect, none of the other ticket priorities need to change

If you need due dates or expected completion dates, you can apply them to milestones – batches of high-priority tickets - and change everything at one time.