9. A method performed by at least one computer system, of retrieving documents in response to a search query that includes one or more phrases, the method comprising: selecting from a corpus of documents a plurality of documents that include at least one of the one or more phrases and that are relevant to the query based on relevance scores for the selected documents; determining, using a processor of the at least one computer system, for each selected document a date range with which the document is associated, the date range representing a period for which no change in the document has been detected; grouping the documents into a plurality of groups using to the date range associated with each document; and within each group, ranking the documents according to relevance scores determined based on relevance data valid for the date range associated with each document.
10. The method of claim 9 , wherein each group represents a month, newer groups are presented ahead of older groups, and within each group, higher relevance scores are presented ahead of lower relevance scores.