To edit the published date of an article, go to the article page and click edit to bring up the overlay. Scroll down to the bottom of the page and click the 'Authoring Information' horizontal tab.
Insert the date you would like the article to have been published on in the 'Authored on' field. Ensure that the date is formatted as instructed on the page. The time zone offset should be left as is.
Click 'Save' at the bottom of the screen.