Google has an option in the advanced search page that lets you restrict the results to pages from the last 3 months, 6 months or a year. Until now, the date was the last time when Google indexed a page. Because this doesn't mean too much and Google crawls the pages pretty fast, it was changed to the date of the first indexing.
As you can see when you search for "Google Maps", the results are still ranked by relevancy, but you won't find (too many) pages that already existed three months ago.
Unfortunately Google doesn't offer a way to define other intervals (at least not in the standard interface), but you can use the daterange operator (daterange:startdate-enddate, where startdate and enddate are Julian dates). This form does the conversion between the Gregorian date and the Julian date for you (based on some code from U.S. Naval Observatory):
{ via Matt Cutts }
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