24. One or more machine-readable tangible and non-transitory media having information for ranking a search result, wherein the information, when read by at least one machine, causes the at least one machine to: access a set of recency ranking data comprising one or more past search queries, one or more past search results, and one or more recency features, wherein the recency features comprise: a time-sensitive feature representing a particular time period that was determined based on content of at least some of the past search queries, wherein the at least some of the past search queries were sensitive to the particular time period, and a query timestamp feature representing the time at which the past search queries were received at a search engine; train a first ranking model via machine learning based on the recency features; and determine when recency is to be utilized for ranking a search result based on the ranking model.
24. One or more machine-readable tangible and non-transitory media having information for ranking a search result, wherein the information, when read by at least one machine, causes the at least one machine to: access a set of recency ranking data comprising one or more past search queries, one or more past search results, and one or more recency features, wherein the recency features comprise: a time-sensitive feature representing a particular time period that was determined based on content of at least some of the past search queries, wherein the at least some of the past search queries were sensitive to the particular time period, and a query timestamp feature representing the time at which the past search queries were received at a search engine; train a first ranking model via machine learning based on the recency features; and determine when recency is to be utilized for ranking a search result based on the ranking model. 25. The media of claim 24 , wherein the software is further operable when executed by the at least one machine to: access a plurality of first search queries and a plurality of first search results, each of the first search results being generated by the search engine for one of the first search queries and comprising one or more first network resources; and for each of the first search queries automatically determine whether the first search query is sensitive with respect to a first time period during which the first search query is received at the search engine; and if the first search query is sensitive, then include the first search query and the first search result generated for the first search query in the set of recency ranking data.