The depublish extension adds an extra date field to your editor's sidebar (entries and pages). If you set an unpublish date, PivotX automatically sets the publish status of the entry or page to "on hold" when the date expires.
This extension only works with PivotX 2.1.0 beta 6 (or later)
Download this extension here: http://extensions.pivotx.net/entry/15/depublish