Aaron Wall, the author of the SEOBook, explains us why we should not use dates in the URLs, one more change for this blog.
Everything you do has a chance to provide a clean or dirty signal of relevancy to search engines and searchers.
While a date in the URL may provide a signal of relevancy to some searchers looking for archived information, many people who are explicitly looking for old information will likely put a date in their search query, which will match page text even if the date is not in the URL. Other searchers have a good possibility of rediscovering the information they read in the past with the help of search personalization algorithms. But most queries are not for old stuff. And that makes me believe that using the date in the URL causes more harm than good.