Google is now officially an adult. The internet giant is celebrating its 18th birthday on Tuesday with an animated Doodle shown to web browsers around the world. The company, founded by Larry Page and Sergey Brin in 1998, traditionally marks its birthday on the Google homepage on September 27. However, not even Google seems to really know when it was formed. Since 2006, it has celebrated its birthday on September 27, but the year before that, had it as September 26. In 2004, its 6th birthday Doodle went online on September 7 and in the year before that, it was September 8. In fact, none of these dates seem to have any particular relevance. The company's own history lists its incorporation date in 1998 - the nearest thing to a corporate birthday - as September 4. And the idea for Google began years earlier in 1996, as a research project at Stanford University, when Page and Brin had a new idea for a search engine that would rank pages by how many other sites linked to it, rather than the crude format others used at the time - ranking them by how often the search term appeared on the page. Google alluded to confusion over its birthday in 2013, admitting it had celebrated its birthday on four different dates, but September 27 now seems to have stuck. The company may have just chosen the date because it was what the first birthday Doodle in 2002 used. Google's 13th birthday Doodle Google has celebrated its birthday with a Doodle every year since its fourth birthday in 2002. However, the first Doodle, for the Burning Man festival in 1998, actually occurred before the company was technically founded. Last year, the company split under a new corporate umbrella, Alphabet, which hived off more outlandish divisions of the company such as its driverless cars unit and robotics subsidiary Boston Dynamics. The change saw its founders move away from Google’s day-to-day running. Larry Page and Sergey Brin in Google's early days Credit: AP 18 years after it was founded at Stanford University, Google is one of the world's most powerful companies. It is second only to Apple in value, with a market capitalisation of $541bn (£417bn). Page and Brin are ranked 12th and 13th on Forbes' list of the richest people in the world, with net worths of $35.2bn and $34.4bn respectively.