Suppose you need the Posted Date updated but are not making any changes to the Opportunity itself. In that case, you can Refresh the Posted Date by clicking the Refresh Now button while in the Edit Opportunity screen.*.
From your home screen, click the "Manage Jobs" button.
On the Manage Jobs screen, click the "Edit" (pencil) icon to edit that Posting.
In the Navigation panel, click the "Refresh" button.
NOTE: Postings of 120 days or older are allowed to be refreshed.
In the Confirmation screen, click the "Save" button.