I haven't noticed it before, but you can add an as_qdr parameter to the end of a URL in Google to get a drop-down menu (next to the search box) that lets you search by date. For example:

http://www.google.com/search?&q=macosxhints&as_qdr=d

So I wrote this bookmarklet that does that automatically:

javascript:window.open(document.URL + "&as_qdr=d")

You can customize it as you please:

• d[number]: past number of days (e.g.: d10)
• w[number]: past number of weeks
• y[number]: past number of years

More info on this can be found in this post on the Google Operating System site.