A user on popular website Reddit has registered Ground Zeroes on Konami Core (Konami’s own reward program), and received the following message:
“Come back early 2015 to claim your downloadable content!”
Kojima said in the recent Twitch interview that Ground Zeroes won’t be getting DLC, because he wants to focus on The Phantom Pain now. That suggests the DLC mentioned is for The Phantom Pain, and also that the game should be out by that time.
In fact, this is probably the same content that comes with the first batch of Metal Gear Solid V: Ground Zeroes copies, since this content will be usable in Mother Base once The Phantom Pain comes out. So things do add up.
Of course this is all just guesswork, but a release of 2015 does seem likely. The question is when.
Source: Reddit, via TheArabGamer