17. A computer system comprising: at least one processor; and a memory storing instructions that, when executed by the at least one processor, cause the computer system to perform operations comprising: selecting from a corpus of documents a plurality of documents that include at least one phrase of a search query received by the computer system and that are relevant to the search query, the selection being based on a relevance score for the selected documents; determining for at least one of the selected documents a date range for the document, the date range representing a period for which no change in the document has been detected; calculating a weighted relevance score for each of the selected documents, the weighted relevance score for a particular document being based on the relevance score for the particular document and a difference between a date the query was submitted and the date range for the particular document; and ranking the selected documents according to their weighted relevance scores.
17. A computer system comprising: at least one processor; and a memory storing instructions that, when executed by the at least one processor, cause the computer system to perform operations comprising: selecting from a corpus of documents a plurality of documents that include at least one phrase of a search query received by the computer system and that are relevant to the search query, the selection being based on a relevance score for the selected documents; determining for at least one of the selected documents a date range for the document, the date range representing a period for which no change in the document has been detected; calculating a weighted relevance score for each of the selected documents, the weighted relevance score for a particular document being based on the relevance score for the particular document and a difference between a date the query was submitted and the date range for the particular document; and ranking the selected documents according to their weighted relevance scores. 20. The system of claim 17 , wherein the date range includes a first value indicating the date the document was indexed and a second value, the second value being a token indicating a currently valid document when the date range includes the date the query was submitted.