I can see there will be some serious fraud going on with the dates as naive webmasters try to get their sites indexed more often. On a page like this do you give the date of the last post, the date of the last signature update, or blog update. The post (obviously) but I imagine there will be some creative answers... Sarah
I'm talking about the lastmod tag that applies to every url https://www.google.com/webmasters/sitemaps/docs/en/protocol.html#xmlTagDefinitions
I'm not sure this would be a big concern for Google. I appreciate this new feature so that I can actually get Googlebot to index certain pages less often. I don't really care if Googlebot only indexes static pages once per month. However pages that change frequently (like a homepage typically does) I would like indexed frequently and thus I will tell Googlebot that through the new XML sitemap interface. Also most new pages are linked to from the homepage so Googlebot should quickly pick up any new content. I assume the same factors are still at work where even if you say a particular page was updated today... but Googlebot doesn't see a major difference when it indexes again... that Googlebot will just start to ignore your last mod dates. That is just speculation though.
I'm not using it. Its hard for my site to do. I might use it later but it doubles the number of SQL queries.
I will use the actual updating date. I don't think there will be advantage of pulling googlebot to spider a page that you have not changed.
They can already track document change so if you trick them into coming back they will instantly notice whether it was indeed updated or not. No point in trying to fool them, they're smarter than that. It'd be a waste of your own bandwidth.