Previously, I believe Google estimated the age of a url as the last time that we fetched that page. Given how quickly Google refreshes its main index, that didn’t mean quite as much recently. Now for date-based search, Google estimates url ages by the first date that we saw a url.
Here’s an example. A normal search for [Google] looks like this:
If you restrict the results to the last three months, the same search for Google gives more recent urls:
Just a handy search tip for power users. Thanks to the Googlers who improved date-based search.