Yeah, I also would have said ~2010. After looking it up it traces back to at least 2008. I remember trying out the leaked alpha version, and it was incredibly unstable, and missing all the core features, and Milestone 1 wasn't any better.

Kinda interesting that they still continued with the project for 10 years, despite most of the core productivity features becoming mainstream during the time, and it not offering that much of an upside compared to the standard Windows UI.