GoogleGuy - 4:08 am on Apr 21, 2004 (gmt 0)

We have a minty-fresh crawl that picks up several million pages pretty much every day. I believe that the date is displayed if a) the page was seen in the minty fresh crawl, and b) the current date is within N days of the crawl date, where N is a small integer such as 2 or 3. :) So if we crawled your page on April 20th in the fresh crawl, then on April 20th, 21st, and maybe April 22nd you'd see the date alongside your search result snippet.

The difference between the "deep crawl" and the "fresh crawl" was much more apparent this time last year when we were only pushing a new deep index about once a month. As Google improved our infrastructure, the deep crawl became much fresher, and the fresh crawl could sometimes go pretty deep. So these days, there's not as much visible difference, but I think currently we only show dates if you were in the fresh crawl and within that window of recency.

There ya go, the mystery of the fresh dates laid bare. Probably no one except for SEOs have ever wondered about the criteria, so it's good that we have this informal way of communicating. :)