It took only a dozen years for humanity to add another billion people to the planet, reaching the milestone of 7 billion Monday — give or take a few months. Demographers at the United Nations Population Division set Oct. 31, 2011, as the "symbolic" date for hitting 7 billion, while acknowledging that it's impossible to know for sure the specific time or day. Using slightly different calculations, the U.S. Census Bureau estimates the 7-billion threshold will not be reached until March.