service branch. For example, the statutory rank of the vice commandant of the Coast Guard was raised to admiral by the Coast Guard Authorization Act of 2015, to align the leadership structure of the Coast Guard to that of the other armed services and recognize the important role of the vice commandant at the national level. The incumbent vice commandant, Vice Admiral Charles D. Michel, was confirmed for promotion to admiral, and assumed the rank on 1 June 2016. The statutory rank of the chief of the National Guard Bureau was raised to general by the National Defense Authorization Act of 2008, with significant congressional support. Lieutenant General Craig R. McKinley, then director of the Air National Guard, was confirmed for promotion to general, and assumed the rank and accompanying office on 17 November 2008. Tour length The standard tour length for most four-star positions is three years, bundled as a two-year term plus a one-year extension, with the following exceptions: The chairman and vice chairman of the Joint Chiefs of Staff serve for a nominal four-year term. Service chiefs of staff serve a nominal four-year term. The chief of the National Guard Bureau serves a nominal four-year term. The director of Naval Nuclear Propulsion serves for a nominal eight-year term. All appointees serve at the pleasure of the president. Extensions of the standard tour length can be approved, within statutory limits, by their respective service secretaries, the secretary of defense, the president, and/or Congress but these are rare, as they block other officers from being promoted. Some statutory limits of tour length under the U.S. Code can be waived in times of national emergency or war. Four-star ranks may also be given by act of Congress but this is extremely rare. Retirement Other than voluntary retirement, statute sets a number of mandates for retirement. Regular four-star officers must retire after 40 years of active commissioned service unless reappointed to grade to serve longer. Reserve four-star officers must retire after five years in grade or 40 years of commissioned service, whichever is later, unless reappointed to grade to serve longer. Otherwise all general and flag officers must retire the month after their 64th birthday. However, the secretary of defense can defer a four-star officer's retirement until the officer's 66th birthday and the president can defer it until the officer's 68th birthday. Officers that served several years in the enlisted ranks