Not just "x hours ago." For older material, you'll see that switch to "x days" and eventually "mmm dd yyyy" format. It's a crawl timestamp, likely also more specifically the last time a cache snapshot was taken. They've shown these in the past.
As a current example, since that linked item is from 2007:
There's a post on a site I manage that was published March 13 this year. If this were based on the publication date, then Google should obviously be showing March 13 there, or at least something very close. Instead the result shows July 07, which is the day a comment eventually got posted to the entry.
It's the only comment, so crawls in the intervening months would've been essentially identical and so not worth noting/caching. It's also probably significant that I have a sitemap template set up for the site that includes the optional
lastmod timestamp, and counts new comments as modifications to the document. Given that the page content was fixed for several months and the new crawl happened the precise day it finally got updated, I'd tend to think the sitemap was involved.