Let's say that I run a forum dedicated to computer hardware. Naturally, people are going to ask questions like:
What is the best laptop for running [os]
What is the best video card for under [amount]
These may be perfectly fine discussions, but the content loses usefulness over time. An answer to either question asked in 2007 might still be relevant in 2008, but definitely not in 2012.
Is there a way that I can tell search engines that certain pages might not give visitors what they're looking for after a certain date, and perhaps hint to a page on my site that would provide good information? Perhaps something I could set in HTTP response headers, meta tags or even a site map?