10. A method, performed by at least one computer system, of retrieving documents in response to a search query that includes a phrase and a first date, the method comprising: accessing an index for a corpus of documents, the corpus comprising documents having multiple instances, each instance of a particular document being acquired at a different date and a new instance being acquired when a change from a previous instance is detected, wherein each instance has an associated date range; selecting, using the index, a plurality of documents that are relevant to the phrase; for each document instance of the plurality of documents: determining, using a processor of the at least one computer system, a difference between the first date and the date range for the instance, and down-weighting a relevance score for the document instance in proportion to the difference; and ranking the document instances according to their respective weighted relevance score.
10. A method, performed by at least one computer system, of retrieving documents in response to a search query that includes a phrase and a first date, the method comprising: accessing an index for a corpus of documents, the corpus comprising documents having multiple instances, each instance of a particular document being acquired at a different date and a new instance being acquired when a change from a previous instance is detected, wherein each instance has an associated date range; selecting, using the index, a plurality of documents that are relevant to the phrase; for each document instance of the plurality of documents: determining, using a processor of the at least one computer system, a difference between the first date and the date range for the instance, and down-weighting a relevance score for the document instance in proportion to the difference; and ranking the document instances according to their respective weighted relevance score. 15. The method of claim 10 , wherein the difference is measured from an open date of the date range.