1. An information estimation device that estimates the time of issue of a document to be estimated, comprising: a processor; a candidate generation unit that extracts time representations described in the document to be estimated, and generates a plurality of issue time candidates potentially corresponding to the time of issue of the document to be estimated based on the extracted time representations; and an issue time estimation unit that obtains temporal proximities between a given issue time candidate and other issue time candidates for each of the plurality of issue time candidates, and estimates the time of issue of the document to be estimated based on the obtained temporal proximities.