David Galbraith writes:
Having just searched for some software on Google I realized that the top results were 4 years old and useless.
It is not technically difficult to create date ordering, but it is computationally expensive and requires comparisons as documents are crawled. There is, however, one area where searching by date is already there: weblog searching.
The model where content sites ping a server when there are updates soves the date problem cheaply and increases overall relevancy. The ping model is as different from the way search engines currently work, as push is to pull.
This is another compelling reason why the search engines will be caught off guard if they do not pay attention to weblog style publishing and ping servers, not just as an additional feature, but a component that is core to what they do.