1. A method performed by at least one computer systems of providing an archival index of a plurality of documents, the method comprising: maintaining an index of a plurality of documents, the index relating phrases contained in at least some of the documents with documents containing the phrases; accessing a document of the plurality of documents; determining, using a processor of the at least one computer system, whether the document has changed since a previous date the document was indexed; responsive to determining that the document has changed, indicating a first date range, including the previous date, for which the document is valid; indexing the document to obtain current information-retrieval relevance data for the document, the current information-retrieval relevance data including phrases contained in a current version of the document; storing the current information-retrieval relevance data in association with a current date range for the document; and maintaining previous information-retrieval relevance data for the document in association with the first date range, the previous information-retrieval relevance data including phrases contained in a version of the document that was valid during the first date range.
1. A method performed by at least one computer systems of providing an archival index of a plurality of documents, the method comprising: maintaining an index of a plurality of documents, the index relating phrases contained in at least some of the documents with documents containing the phrases; accessing a document of the plurality of documents; determining, using a processor of the at least one computer system, whether the document has changed since a previous date the document was indexed; responsive to determining that the document has changed, indicating a first date range, including the previous date, for which the document is valid; indexing the document to obtain current information-retrieval relevance data for the document, the current information-retrieval relevance data including phrases contained in a current version of the document; storing the current information-retrieval relevance data in association with a current date range for the document; and maintaining previous information-retrieval relevance data for the document in association with the first date range, the previous information-retrieval relevance data including phrases contained in a version of the document that was valid during the first date range. 9. The method of claim 1 , further comprising: deriving a unique document identifier for an instance associated with a date range for a document from a date included in the date range and from a document locator for the document.