1. A computer-implemented method for determining dates for date-qualified query terms within search queries, the computer-implemented method comprising: maintaining data that indicates, for a plurality of query terms, whether the plurality of query terms are date-qualified query terms; receiving a query at a search engine; in response to receiving the query, inspecting the query to determine that (a) the query contains a particular date-qualified query term, and (b) the query does not explicitly identify a date for the particular date-qualified query term; and for the query, determining at least one date with which to associate the date-qualified query term based, at least in part, on queries previously received by the search engine.