In an interview with Famitsu, as relayed by Polygon, Hideo Kojima explains how Ground Zeroes will ease players into the world of Metal Gear Solid V.

In case there was still any confusion as to the time line for Ground Zeroes and The Phantom Pain, Kojima confirmed that GZ will take place in 1975, one year after the events of Peace Walker. The bulk of the game, The Phantom Pain, will take place in 1984. You will more than likely need to play GZ in order to understand the story and motivations for Big Boss in The Phantom Pain. Naturally, the events in this game should lead into the start of the series, Metal Gear.