Maybe you already know about this, I discovered this only today (thanks Claudio).
Try this: go to google.com, perform a search, then in the address bar add to the querystring the following:
you will obtain the same page, but with a new dropdown list right before the "Search" button. With this, you have the chance to restrict the time span of your query!
For example, you can have Google return results only from the last past week!!!
Now this is a great idea, for some kind of searches.