Filtering google searches by time periods has been one of the most frequently requested features. By manipulating the search url, you can trigger google's filter by data dropdown box to appear. With this drop down box, you can easily select what time dates you want google to filter the results.
I often want to search for updated news on a common search term. For example, say you wanted to search for Obama news for the last 24 hrs. Using this little google hack, you can focus your search over whichever time period you want.
First goto google.com and search for your search term. I'm going to use "obama" as my example.
Obviously, I receive the highest ranked searches all-time for Obama.
Next, just add &as_qdr=d to the end of the URL and press ENTER.
This post was suggested by techcrunch who also points out that more focus can be obtained: Just change the =d to d5 for 5 days, or w5 for five weeks, or y5 for 5 year, etc.