What meta tag should be used to have a date appeared in search results? Or does Google automatically determines it?
For example if you search for the term "adsense tips", some search results has a date in it something like: Title here 10 Feb 2012 - Description goes here
if only we know wtf you were talking about... if you mean the date at when google cached your site, look at the date at the captcha
He's talking about the dates in the excerpts shown in Google, right? Those are usually there because the date is shown on the page of the sites in question. If you're using wordpress and your theme shows the date of the post and Google thinks that is relevant info for searchers it will be included automatically.
Google determines it automatically. I have seen showing up on one of my sites for wordpress pages as well as posts, which do not have the publish date displayed. I think in that case they might be pulling the date of the first comment or something? Some of my pages have it and other don't so I really don't know when they decide to show it. I think all of my posts have it though.
Just found a code in my site (I didn't code it) where it has a <time>. Could it be the reason too? Ref: http://www.brucelawson.co.uk/2012/best-of-time/
If you want to include a date for your web page you can put the same either in title or in description.