1. A computer-implemented method, comprising: receiving a search query from a client; determining whether real-time search results should be included in a response to the search query; if real-time search results should be included, generating a user interface document that includes client software that, when executed on the client, causes the client to: receive real-time search results for the search query and a time token, wherein the time token is data that identities a most-recent time that any resource identified by any of the received real-time search results was updated, present the real-time search results for the query, re-submit the search query with the time token, obtain additional real-time search results that are more recent than the most-recent time identified by the time token, wherein an additional real-time search result is more recent than the most-recent time identified by the time token when a resource identified by the additional real-time search result was last updated more recently than the most-recent time identified by the time token, and present the additional real-time search results; if real-time search results should not be included, generating a user interface document that does not include the client software; and sending the user interface document to the client as a response to the search query.