20. The system of claim 19 , wherein to train the first ranking model using the recency features and the relevance features comprises: for each of the past search queries, determine a ranking score for each of the recency network resources comprised in the past search result generated for the past search query using a second ranking model that has been trained without any recency feature; construct the relevance features from the ranking scores; and apply the recency features and the relevance features to the first ranking model via machine learning.
19. The system of claim 13 , wherein to train the first ranking model via machine learning comprises train the first ranking model using the recency features and a plurality of relevance features.