3 months:
http://www.google.com/search?as_qdr=m3&as_q=***

6 months:
http://www.google.com/search?as_qdr=m6&as_q=***

One year:
http://www.google.com/search?as_qdr=y&as_q=***

You have to enter in your search query in the place of “***”, of course. I tried modifying the “as_qdr” parameter for other lengths of time, but it appears that Google only supports these three specific date limitations. It won’t even accept “12m” or “1y” as a parameter; a one-year search limitation can only be specified, with no numeral, as “y”.

On OSX, these three searches can be directly used with Quicksilver. If the URLs are added to a browser’s bookmarks (and the plugin for that browser is loaded in Quicksilver), QS automatically senses that these bookmarks are searches and allows your search term to replace the “***” string in the URL. The trick is to name the bookmarks properly so that you can find them easily with Quicksilver.

Unfortunately, Quicksilver can’t use these URLs as searches if you put them in del.icio.us and access them with the del.icio.us plugin; the ampersand (”&”) character confuses the del.icio.us plugin and nothing happens when you select the bookmark.