I didn't get 400 days. I just wonder if it really matters or if they're just saying that to sound cool.
It says Your choices made in Season 1 and the "400 Days" DLC will affect the decisions in Season 2. Although "400 Days" is not a direct part of the story thread joining Seasons 1 and 2, the player's actions in "400 Days" will affect some elements of Season 2. If you haven't played "400 Days", the elements that would have been affected will be seamlessly pre-determined by the game.