1. A method implemented by data processing apparatus, the method comprising: serving, to a user device, a document that includes value text that specifies a first value that is not tagged as a time sensitive attribute value; automatically identifying an entity based on entity text included in document text of the document after serving the document to the user device; automatically, after the document is served to the user device, identifying a time-sensitive attribute for the entity specified by attribute text included in the document text, wherein attributes identified by the document text have not previously been identified for the document as time-sensitive before serving the document; identifying the first value for the time-sensitive attribute based on the value text included in the document text; providing data to the user device that causes a prompt to be displayed in the document, the prompt identifying the first value for the time-sensitive attribute and including a user-selectable interface element that, upon selection, will cause the first value for the time-sensitive attribute to be tagged as a time-sensitive attribute value; and generating, responsive to a selection of the user-selectable interface element, a tag for the first value for the time-sensitive attribute in response to receiving user input indicating a selection of the user-selectable interface element, the tag indicating that the first value is time-sensitive, and wherein the tag causes the value text specifying the first value to be updated in response to one or more predetermined actions; responsive to generating the tag, generating a query specifying the entity; providing the query to a search system that provides a result value for the time-sensitive attribute of the entity included in the query; providing, to a user device that is currently accessing the document, result data that causes presentation of the result value as a replacement for the first value as indicated by the tag.