Observed on July 11 every year since 1989.
From Census.gov > Data > Data Tools > U.S. and World Population Clock > Notes on the World Population Clock:
The populations displayed on the clock are not intended to imply that the population of the world is known to the last person. Rather, the clock is the Census Bureau's estimate of the world population size and an indication of how fast it is growing. According to current estimates, the world population reached 7 billion in 2011 and the new projections indicate that the 8 billion marker will be reached in 2023.
Note: The International Database (IDB) was developed by the Census Bureau to provide access to accurate and timely demographic measures for populations around the world. The IDB provides estimates and projections for 227 countries and areas which have populations of 5,000 or more and as recognized by the U.S. Department of State. The IDB estimated the world population reached 7.5 billion on April 16, 2017.
From Census.gov > Our Surveys & Programs > International Programs Center:
The U.S. Census Bureau conducts demographic, economic, and geographic studies of other countries and strengthens statistical development around the world through technical assistance, training, and software products.
Note: Visit the IDB Interactive Tool Help Page for more information.
Source: Our World in Data. “The absolute annual change of the population. This visualization here shows the annual global population increase from 1950 to today and the projection until the end of this century. The absolute increase of the population per year has peaked in the late 1980s at over 90 million additional people each year. But it stayed high until recently. From now on the UN expects the annual increase to decline by around 1 million every year.”
Citation: Max Roser, Hannah Ritchie and Esteban Ortiz-Ospina (2013) - "World Population Growth". Published online at OurWorldInData.org. Retrieved from: 'https://ourworldindata.org/world-population-growth' [Online Resource]