Quote Originally Posted by Vino Reign View Post
well when you get an achievement it logs the date you got it so they will know
Actually offline achievements do not get a time stamp. Due to people being able to change their time MS made it so only online achievements get time stamps. However even if you did do it offline you would not be able to log into Xbox Live until the game was released, since Bungie is doing checks for any achievements that have been unlocked before the release date.