The U.S. Space Force traces its roots to the beginning of the Cold War, with the first Army Air Forces space programs starting in 1945. In 1954, the Western Development Division, under General Bernard Schriever, was established as the first dedicated space organization within the U.S. Armed Forces and continues to exist as the Space Force's Space and Missile Systems Center. Military space forces were organized under several different Air Force major commands until they were unified when Air Force Space Command was established on 1 September 1982. U.S. space forces first began conducting combat support operations in the Vietnam War and continued to provide satellite communications, weather, and navigation support during the 1982 Falklands War, 1983 United States invasion of Grenada, 1986 United States bombing of Libya, and 1989 United States invasion of Panama. The first major employment of space forces culminated in the Gulf War, where they proved so critical to the U.S.-led coalition that it is sometimes referred to as the first "space war".

The first discussions of creating a military space service occurred in 1958, and it was nearly established in 1982 by President Ronald Reagan as part of the Strategic Defense Initiative, but the idea dissolved with treaty compliance concerns. A 2001 Space Commission argued for the creation of a Space Corps around 2007–2011, but no action was taken by the Obama Administration. On 20 December 2019, the United States Space Force Act was signed into law as part of the National Defense Authorization Act, reorganizing Air Force Space Command and other space elements into the United States Space Force, and creating the first new independent military service since the Army Air Forces were reorganized as the U.S. Air Force in 1947.
From the passage below, please extract the official starting date of the United States Space Force.
The United States Space Force Act was signed into law as part of the National Defense Authorization Act on December 20, 2019. This act reorganized the Air Force Space Command and other space elements into the United States Space Force, and officially created the first new independent military service since the Army Air Forces were reorganized as the U.S. Air Force in 1947.