The date was provided during a special event showcasing the game. You can check out a trailer for the game below.
Gran Turismo Sport was first announced last October. At the time, there were plans for a beta version to be available in early 2016. There's no mention of that in today's announcement, nor is there any reference to the PlayStation VR support that was mentioned last year.
Sport is not to be confused with Gran Turismo 7, which is a separate game. Sony's Jim Ryan told Eurogamer last year that GT7 wasn't being announced yet -- this despite Yamauchi suggesting in 2013 that GT7 would be out in 2014 or 2015.