20. The method of claim 16 , further comprising: deriving a first unique document identifier for an instance of the accessed document associated with a the new prior date range for a document from a date included in the prior date range and from a document locator for the accessed document; and deriving a second unique document identifier for an instance of the accessed document associated with a the new current date range for a document from a date included in the new current date range and from a document locator for the accessed document.
16. A method performed by at least one computer system of providing an archival index, 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, each document associated with a current date range for which the document is currently indexed and associated with current information-retrieval relevance data for the current date range, wherein the current date range includes a first date, and at least one document associated with a plurality of non-overlapping, contiguous date ranges prior to the current date range, each prior date range having information-retrieval relevance data for the document for that date range; accessing a document of the plurality of documents that has been previously indexed and having current information-retrieval relevance data; responsive to the accessed document having changed since the first date, updating, using a processor of the at least one computer system, the current date range for the changed document to be a new prior date range and to include a closed date; storing the current information-retrieval relevance data for the changed document as information-retrieval relevance data for the new prior date range; establishing, using a processor, a new current date range for the changed document, wherein the new current date range includes a current date as an open date; updating the current information-retrieval relevance data for the changed document, the updated current information-retrieval relevance data including phrases contained in the changed document; and storing the updated current information-retrieval relevance data in association with the new current date range.