It depends! Within days of Skyrim releasing in 2011, modders from TESR Morroblivion started tinkering with the new engine. With the release of the Creation Kit on February 7th, 2012, Morroblivion porting began in earnest.
Six months later, Skywind had a name and logo. So if you want to bake us a cake, the 2nd August is as good a date as any!
Around 2016, the nature of the project morphed into a game that will recreate every asset, from texture to mesh to sound. This was a gradual change, not a pivotal decision on a specific date. That leads us to the complete remake project that Skywind is today.