instruction: When did Phil Knight announce he would step down as chairman of Nike
context: Throughout the 1980s, Nike expanded its product line to encompass many sports and regions throughout the world. In 1990, Nike moved into its eight-building World Headquarters campus in Beaverton, Oregon. The first Nike retail store, dubbed Niketown, opened in downtown Portland in November of that year.  Phil Knight announced in mid-2015 that he would step down as chairman of Nike in 2016. He officially stepped down from all duties with the company on June 30, 2016.  In a company public announcement on March 15, 2018, Nike CEO Mark Parker said Trevor Edwards, a top Nike executive who was seen as a potential successor to the chief executive, was relinquishing his position as Nike's brand president and would retire in August.
response: Phil Knight announced he would step down in 2015 as chairman and offically stepped down in 2016