Internet search giant Google is celebrating its 21st birthday.

The real date when Google was founded, though, remains something of a mystery – and even the company admits it does not know exactly when it began.

Google, which was founded by Stanford students Larry Page and Sergey Brin in 1998, usually marks its birthday on September 27 each year, although in 2005 it marked the anniversary a day earlier.

To add to the confusion, Google sent out their celebratory doodle on September 8 in 2003, and a day earlier in 2004.

When was Google founded?

The company lists its start date as September 4, 1998 on its website but the idea for creating the new search engine first materialised in 1996.

September 27 appears to have stuck through chance, because that’s the date when the company first celebrated its birthday, back in 2002.

Page and Brin derived the title from the word ‘googol’, which is the number one followed by a hundred zeroes – a reflection of the vast amount of searchable data.