One year ago today, May 7, 2016, the clock projected an estimated population of 322,776,678.
About the Population Clock Projection (via U.S. Census):
Short-term projections for the resident population of the United States use a monthly series of population estimates starting with the April 1, 2010, resident population from the 2010 Census. These projections do not include members of the U.S. armed forces overseas or other U.S. citizens residing outside the United States.
See Also: U.S. Population Projected to Cross 325 Million Threshold (via U.S. Census)
Source for some material in this post.
Direct to U.S. and World Population Clocks