The U.S. Census Bureau's population clock does not magically subtract who has died or left the country in the last minute nor does the agency get a call from the hospital every time a baby is born. Instead the population clock ticks off the resident count based on the monthly national population estimates, and the Census Bureau assumes daily population change is constant. This includes the rates of births, deaths, international migration and Armed Forces movement. Growth rates for each month take into consideration changes in each. For example, death rates in the winter months are slightly higher than the death rates at other times of the year.
To produce the national population estimates, the Census Bureau uses a demographic accounting method. First, the Census Bureau adds the number of children who were born and subtracts the number of people who have died since the last census. This information is provided by the National Center for Health Statistics and includes other information such as race and place of birth. The estimates also take into account the number of people moving in and out of the United States such as international migrants, people in the military and U.S. residents moving abroad. The Census Bureau uses the American Community Survey to measure international migration and the various branches of the military, and the Department of Defense provides information on the movement of the military population.
The population estimates do not include tourists or other people visiting the United States for just a short time, people stationed overseas or U.S. citizens who live abroad.
For more information about the Census Bureau's methodology for producing the national estimates used to update the U.S. population clock, visit the Population and Housing Estimates Web Site.