One day this month will mark Google’s official 10th birthday. The company will probably celebrate with a blog post and a special logo and we’ll see a slew of articles about how much they’ve accomplished in those 10 years. A few publications couldn’t wait and got their Google tributes out last month.

But when exactly should we be celebrating? Almost certainly on September 27. But the real answer is way more complicated than that.

Google is actually nearly 13-years-old, if you go by their corporate history page: “By January of 1996, Larry and Sergey had begun collaboration on a search engine called BackRub, named for its unique ability to analyse the ‘back links’ pointing to a given website.”

But if you go by when the Google.com domain name was registered, September 15 1997, the company will turn 11 next week.

However, the date Google celebrates as their birth month is a year later, September 1998. They celebrated on September 7, the date of the company’s incorporation, until 2005. Since 2005 (and also randomly in 2002), they’ve celebrated on September 27.

So why do they celebrate it on the 27th? According to Search Engine Land’s Danny Sullivan, who wrote about this mess last year, they pushed the date forward in 2005 to allow for the announcement of an index-size milestone (ie the record-breaking number of pages the search engine was sifting through).

At least Google is consistently inconsistent: “Google opened its doors in September 1998. The exact date when we celebrate our birthday has moved around over the years, depending on when people feel like having cake.”

So Happy 10th, 11th and/or almost 13th birthday Google! The first three people who sing Happy Birthday to Google in a video comment below (all the way through, full volume, go for it) get a TechCrunch T-shirt.