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I started noticing that for some search results by google, there is a "x hours ago" before the description under the link. How does google determine this?

There is also this link "Get more results from the past 24 hours". How does this work?

Is that based on last crawl timestamp? if so, how can one make google crawl a dynamic site more frequently? should we set that in HTTP headers like last-modified?

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1 Answer 1

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.

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I'm not sure whether that is the only factor. Because for certain searches, some sites tend to show up with "x hours ago" constantly. I maintain a site that is also updated constantly every 2 hours and the sitemap "changefreq" is set to hourly, BUT my site in search results doesn't indicate this text. If indeed you are right, how can one make google determine the crawl frequency, I already tried this using webmasters but no luck. –  narxgun Aug 7 '11 at 16:51
    
@narxgun Didn't say it was the only one; almost nothing with Google ever has a single factor. I gave you two above, in fact, though depending how much you assume they use sitemaps they might seem the same. The sitemap example's just the one that I can actually(almost) prove. Nobody can respond to your comment about sites always showing "x hours" without examples. Also note I said lastmod, not changefreq. Changefreq's a hint how often a document might change(cf comment in the protocol doc); lastmod's the actual timestamp of a definite change. –  Su' Aug 7 '11 at 17:18

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