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

full_context: Document 1: Kirstjen Nielsen: 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.
Document 2: United States Secretary of Homeland Security: 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.
Document 3: John Marshall Mitnick: John Marshall Mitnick is an American attorney who serves as Senior Vice President, General Counsel, and Secretary of the Heritage Foundation. He is President Donald Trump's nominee to become General Counsel of the United States Department of Homeland Security. Mitnick previously served as Vice President, General Counsel, and Secretary at Raytheon. During the presidency of George W. Bush, he served as Deputy Counsel of the Homeland Security Council and then as Associate Counsel to the President.
Document 4: Elaine Chao: Elaine Lan Chao (; born March 26, 1953) is an American politician who is the 18th and current United States Secretary of Transportation. A member of the Republican Party, she was previously a cabinet member in the administration of George W. Bush.
Document 5: White House Counsel: The White House Counsel is a staff appointee of the President of the United States whose role is to advise the President on all legal issues concerning the President and his Administration. The current White House Counsel is Don McGahn.
Document 6: Kevin Haugrud: Kevin "Jack" Haugrud was the acting United States Secretary of the Interior, serving since the end of the Obama administration on January 20, 2017 until the swearing in of Ryan Zinke on March 1, 2017. Until becoming Acting Secretary, Haugrud was deputy solicitor of the Department of the Interior.
Document 7: K. T. McFarland: Kathleen Troia "K.T." McFarland (born July 22, 1951) is an American government official and commentator who is the nominee to become the next United States Ambassador to Singapore. McFarland served as Deputy National Security Advisor to President Donald Trump during the first months of 2017.
Document 8: Kellyanne Conway: Kellyanne Elizabeth Conway (née Fitzpatrick; born January 20, 1967) is the current Counselor to U.S. president Donald Trump. She is the first woman to ever run a successful presidential campaign. She has been a Republican Party campaign manager, strategist, and pollster, and was formerly president and CEO of The Polling Company Inc./Woman Trend.
Document 9: Rick Dearborn: Rick A. Dearborn is currently the White House Deputy Chief of Staff for Legislative, Intergovernmental Affairs and Implementation and formerly the executive director of Donald Trump's presidential transition team. Dearborn has worked for six U.S. Senators, including two members of Senate leadership, and spent more than 25 years working on Capitol Hill. He was nominated by President George W. Bush and confirmed by the Senate to become the Assistant Secretary of Energy for Congressional Affairs, where he worked with the Senate, House and Tribal Governments on achieving President George W. Bush's Energy Agenda. Rick Dearborn was a member of Donald Trump's presidential transition team, a group of advisors tasked with recommending presidential appointments for the incoming administration. Dearborn had previously worked as the Chief of Staff for Senator Jeff Sessions from December 2004 until 2016. Dearborn, along with Senior Advisor Stephen Miller, is one of two former senior Sessions staffers to have a senior role in Trump's White House.
Document 10: Elizabeth Erin Walsh: Elizabeth Erin Walsh is an American government official who currently serves as Assistant Secretary of Commerce and Director General of the United States Commercial Service. Prior to assuming her current role, she was special assistant to the U.S. President and associate director for presidential personnel. During the presidential transition of Donald Trump, Walsh was part of the landing team for the United States Department of State, helping to fill vacancies at that department and at the United States Agency for International Development.
Document 11: John F. Kelly: 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.
Document 12: Elaine Duke: 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.
Document 13: Kristie Canegallo: Kristie Canegallo was the Deputy White House Chief of Staff for Implementation for U.S. President Barack Obama. Her responsibilities included the execution of major projects such as health care reform, education, and the war in Afghanistan.
Document 14: Homeland Security Advisor: 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.
Document 15: Tom Bossert: Thomas P. "Tom" Bossert (born March 25, 1975) is an American lawyer and Republican White House staffer, currently serving President Donald Trump as Homeland Security Advisor. Immediately before, he was fellow at the Atlantic Council and prior to that he served as Deputy Homeland Security Advisor to President George W. Bush. In that capacity, he co-authored the 2007 National Strategy for Homeland Security. Prior to that, Bossert held positions in the Federal Emergency Management Agency, the Small Business Administration, the Office of the Independent Counsel, and the House of Representatives. He also was appointed as the Director of Infrastructure Protection under Bush, overseeing the security of critical U.S. infrastructure, a post he held for two years. Following that, he was appointed the Senior Director for Preparedness Policy within the Executive Office of the President.
ground_truth: July 31, 2017