Think of end dates as soft deadlines or artificial due dates. 

Some things in life have a fixed deadline with some immediate consequences (e.g. filing taxes, cancel a subscription, exams, assignments etc.).

But there are also other things we just like to have completed by a certain date. Instead of using due dates for both, use end dates for any self-imposed deadlines. 

When creating task or project or editing the title of an item, write "ends "  or "=" followed by the date to add an end date:

Alternatively, click the edit icon on the right of the item and pick your end date from the calendar.

We have some neat planning features planned where end dates will help you see if your plans are realistic.