Date of publishing in search results

From where do search engines retrieve the publishing date that often appears in search results snippets for articles, blogs and forum topics?

Does it come solely from the text on the page, or elsewhere, such as a meta tag or commented text? I have seen some pages that use a pd meta tag, but that seems to be a non-standard tag, so I’m assuming that it is not used in this way.

Is there a particular date format or location in the mark up that is preferred and/or more likely to allow search engines to discover the publish date?

I sometimes see pages listed with their publish dates when they do not show the date in plain text on the page, while some other pages lack any date in their results snippets.

Static sites don’t tend to display published dates, whereas CMSs do. CMSs tend to display dates on the page in easily identifiable areas of the page (the “usual places”) so that’s where I would imagine search engines are getting them from.

I have a Wordpress “blog” whose template I’ve changed to remove the published date: pages from that site don’t display a published date in the search engines.

Rich Data Snippets on forums is explained well here: http://www.seobythesea.com/?p=3419

More info on snippets and how they have changed can be read here: http://searchengineland.com/anatomy-of-a-google-snippet-38357