January 9, 1998: Google’s filed for its PageRank patent on that day.
Patents expire 20 years after the filing date, or January 9, 2018 for this patent.
Shortly thereafter, all the search engines will probably deploy some kind of PageRank algorithm. This will start the slow erosion of Google as the top dog of search engines. PageRank isn’t the only determinant of search result quality, but PageRank made Google.