Google's doodle celebrates 15 years of the company Friday. But while the search giant has celebrated Sept. 27 as its birthday for the past few years, it's not entirely clear where that date comes from. In fact, they've used some other dates in the past, and there are a few days that could arguably make a better claim to the title.

Google incorporated on Sept. 4, 1998, but the domain name was registered back on Sept. 15, 1997 by their own account. And Google doodles haven't been entirely consistent on the birthday. The first birthday doodle, celebrating their fourth birthday, used the 27th in 2002.

But the next year, the birthday doodle was on Sept. 8, while in 2004 they celebrated it on Sept. 7. And in 2005 they celebrated on Sept. 26. But by 2006 they were back to using the 27th, and have stuck with that date since.