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; prior to returning results for the query, performing: 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 time for the particular date-qualified query term; and for the query, determining at least one date that is not in the query with which to associate the date-qualified query term based, at least in part, on queries previously received by the search engine that include both (a) the date-qualified query term, and (b) a date; processing the query based, at least in part, on the at least one date that was not in the query; returning, based at least in part on the at least one date that was not in the query, results for the query; wherein the computer-implemented method is performed by one or more computing devices.