The world's population reached 7 billion on October 31, 2011—if you put your money on the United Nations. However, if you have more confidence in the calculations of the U.S. Census Bureau, the milestone won't be achieved until March 12, 2012. But dates should have an asterisk because neither agency claims to have arrived at the exact date. Indeed, the UN Population Division acknowledges that October 31, 2011, is a "symbolic date." Nevertheless, the UN considers its calculations to be the "gold standard" of population estimates. It arrives at its figures by collecting and analyzing the most recent censuses, surveys, vital statistics and population reports from every country and territory in the world—a total of 228. The margin of error of such calculations is between 1% and 2%. So if you consider a 1% margin of error for the entire world, the date could be off by six months in either direction. The Census Bureau collects the same data as the UN. The discrepancy in the date results from different methods of interpreting the data.
Despite declining death and fertility rates (number of children expected to be born per woman if all women lived to the end of their childbearing years) in many regions of the world—the fertility rate in the U.S. is 2.06, down from 3.8 in 1960, and in India, the rate decreased from 6 in 1950 to 2.6 in 2010—the world's population continues to grow. Population increases occur because the death rate has grown at a faster rate than the fertility rate, creating an annual surplus in births over deaths. In addition, the fertility rate in many developing nations is still quite high: Burundi, Somalia, and Uganda all hover around 6, for example. According to the Census Bureau, about 367,000 people are born each day globally and 153,000 die.
Although the jump from 5 billion to 6 billion and from 6 billion to 7 billion both took 12 years, demographers at the Census Bureau expect the fertility rate to continue to decease each year and thus project the leap from 7 billion to 8 billion will take a bit longer—14 years.