Question: When did the former U.S. Secretary of Homeland Security assume the office of White House Chief of Staff?

Context articles: United States Secretary of Homeland Security
Kirstjen Nielsen
Situation Room
James Baker
Homeland Security Grant Program
Daniel Kaniewski
Stephen W. Rochon
John F. Kelly
Elaine Duke
Homeland Security Advisor

The United States Secretary of Homeland Security is the head of the United States Department of Homeland Security, the body concerned with protecting the U.S. and the safety of U.S. citizens.
 The secretary is a member of the President's Cabinet.
 The position was created by the Homeland Security Act following the attacks of September 11, 2001.
 The new department consisted primarily of components transferred from other cabinet departments because of their role in homeland security, such as the Coast Guard, the Federal Protective Service, U.S. Customs and Border Protection (which includes the Border Patrol), U.S. Immigration and Customs Enforcement (which includes Homeland Security Investigations), the Secret Service, and the Federal Emergency Management Agency (FEMA).
 It did not, however, include the FBI or the CIA.
Kirstjen M. Nielsen is a government official and national security expert who currently serves as Principal Deputy White House Chief of Staff to U.S. President Donald Trump since September 6, 2017.
 She informally performed the role of Deputy Chief of Staff since the time John F. Kelly became White House Chief of Staff on July 31, 2017.
 Nielsen had previously served as Kelly's Chief of Staff at the Department of Homeland Security.
 She is the founder and former President of Sunesis Consulting.
 Prior to serving in the Trump administration, she was a senior fellow and member of the Resilience Task Force of the Center for Cyber & Homeland Security think tank at the George Washington University and served on the Global Risks Report Advisory Board of the World Economic Forum.
 She holds degrees from the Georgetown School of Foreign Service and the University of Virginia Law School.
The Situation Room, officially as the John F. Kennedy Conference Room, is a conference room and intelligence management center in the White House.
 It is run by the National Security Council staff for the use of the President of the United States and their advisors (including the National Security Advisor, the Homeland Security Advisor and the White House Chief of Staff) to monitor and deal with crises at home and abroad and to conduct secure communications with outside (often overseas) persons.
 The Situation Room is equipped with secure, advanced communications equipment for the President to maintain command and control of U.S. forces around the world.
James Addison "Jim" Baker III (born April 28, 1930) is an American attorney and political figure.
 He served as White House Chief of Staff and United States Secretary of the Treasury under President Ronald Reagan, and as U.S. Secretary of State and White House Chief of Staff under President George H. W. Bush.
Homeland Security Grant Program (HSGP) is a program in the United States established in 2003 and was designated to incorporate all projects that provide funding to local, state, and Federal government agencies by the Department of Homeland Security.
 The purpose of the grants is to purchase surveillance equipment, weapons, and advanced training for law enforcement personnel in order to heighten security.
 The HSGP helps fulfill one of the core missions of the Department of Homeland Security by enhancing the country's ability to prepare for, prevent, respond to and recover from potential attacks and other hazards.
 The HSGP is one of the main mechanisms in funding the creation and maintenance of national preparedness, which refers to the establishment of plans, procedures, policies, training, and equipment at the Federal, State, and local level that is needed to maximize the ability to prevent, respond to, and recover from major events such as terrorist attacks, major disasters, and other emergencies.
 The HSGP's creation stemmed from the consolidation of six original projects that were previously funded by the Office of State and Local Government Coordination and Preparedness.
 The HSGP now encompasses five projects in the program: State Homeland Security Program, Urban Areas Security Initiative, Operation Stonegarden, Metropolitan Medical Response System Program, and Citizen Corps Program.
 During the 2010 fiscal year, the Department of Homeland Security will spend $1,786,359,956 on the Homeland Security Grant Program.
Daniel J. Kaniewski is the Deputy Administrator for National Preparedness at the Federal Emergency Management Agency (FEMA).
 Prior to being confirmed by the United States Senate for his role at FEMA, Kaniewski was vice president for global resilience at AIR Worldwide, a catastrophe risk modeling and consulting services firm, and a senior fellow at George Washington University's Center for Cyber and Homeland Security.
 He has also served as the Mission Area Director for Resilience and Emergency Preparedness/Response at the Homeland Security Studies and Analysis Institute and as an adjunct assistant professor at the Georgetown University School of Foreign Service.
 Earlier in his career, Kaniewski served on the White House staff, first as Director of Response and Recovery Policy and later as Special Assistant to the President for Homeland Security and Senior Director for Response Policy.
 Kaniewski began his career in homeland security as a firefighter and paramedic.
Rear Admiral Stephen W. Rochon is the former Director of the Executive Residence and White House Chief Usher.
 He was the first African-American White House Chief Usher.
 Admiral Rochon served his last day on active duty with the Coast Guard on March 9, 2007, and began his service at the White House on March 12.
 Admiral Rochon succeeded Gary J. Walters, who retired in January 2007 after 20 years as White House Chief Usher.
 He resigned as Chief Usher in 2011 to work in the United States Department of Homeland Security and was replaced as Chief Usher by Angella Reid on October 5, 2011.
John Francis Kelly (born May 11, 1950) is the current White House Chief of Staff for U.S. President Donald Trump, previously serving as U.S. Secretary of Homeland Security in the same administration.
Elaine Costanzo Duke (born 1958) is an American civil servant who is the current Acting United States Secretary of Homeland Security.
 On January 30, 2017, she was nominated by President Donald Trump to become the United States Deputy Secretary of Homeland Security under John F. Kelly.
 She became acting Secretary of Homeland Security on July 31, 2017, when John F. Kelly assumed the office of White House Chief of Staff.
The Assistant to the President for Homeland Security and Counterterrorism, commonly referred to as the Homeland Security Advisor and occasionally holding the title of Deputy National Security Advisor for Homeland Security and Counterterrorism, is a senior aide in the Executive Office of the President, based in the West Wing of the White House, who serves as the chief in-house advisor to the President of the United States on homeland security and counterterrorism issues.
 The Homeland Security Advisor is a statutory member of the Homeland Security Council.
 Serving at the pleasure of the President, the Homeland Security Advisor does not require Senate confirmation for appointment to the office.