1. A method, performed by one or more server devices, of determining the freshness of a first document, comprising: identifying, by a processor of the one or more server devices, a set of second documents that each contains a link to the first document; determining, by a processor of the one or more server devices, a freshness attribute associated with each document of the set of second documents, where the freshness attribute indicates when each document was last modified; assigning, by a processor of the one or more server devices, a freshness score to the first document based on the freshness attribute associated with each document of the set of second documents; and ranking, by a processor of the one or more server devices, the first document with respect to at least one other document based on the freshness score assigned to the first document.