Are you playing the DayZ alpha available through Steam Early Access? It wouldn't be a bad assumption, because Bohemia Interactive reports one million sales of the game only four weeks after it became available on Steam. CEO Marek Španěl announced the news via Twitter today.
Congratulations to Bohemia and creator Dean Hall are in order, but let's also take a moment to acknowledge how completely weird the PC gaming business is right now. We're talking about an alpha build of a game that is a standalone version of a mod. Not only that, Hall also made it explicitly clear, with multiple warnings and in all capital letters at the top of the game's Steam page, that it is probably going to break as you play it. A lot.
This is still the first thing you read when you go to the game's Steam page:
"WARNING: THIS GAME IS EARLY ACCESS ALPHA. PLEASE DO NOT PURCHASE IT UNLESS YOU WANT TO ACTIVELY SUPPORT DEVELOPMENT OF THE GAME AND ARE PREPARED TO HANDLE WITH SERIOUS ISSUES AND POSSIBLE INTERRUPTIONS OF GAME FUNCTIONING."