A field called 'Due date' (end date) is used for the end date of an issue. Make sure that you have included that field in your Issue Field Configuration.

The Issue Field Configuration can be found in 'Account Settings --> Configuration --> Issue Fields'.

You need to have Administration rights to have access to the Account Settings section. After that field is included you can easily set the Due date in the "Edit issue"-page.

If you use iterations/sprints/milestones for your project you normally don't set the due date on a task, but the end date for the iteration/sprint itself act as a deadline for all tasks included