15 U.S. Code Subchapter IX - Standard Time defines the time zones of the United States. The Secretary of Transportation
is authorized and directed to foster and promote widespread and uniform adoption and observance of the same standard of time within and throughout each such standard time zone.
As described in section 261, the definition of the time zones is
for the convenience of commerce and the existing junction points and division points of common carriers engaged in interstate or foreign commerce
Congress used their powers to regulate interstate and foreign commerce as the basis to regulate time zones.
In 2005, an amendment was made to the act, in section 260a, which allows states to exempt themselves from "the advancement of time," that is, changing times for daylight savings:
(1) any State that lies entirely within one time zone may by law exempt itself from the provisions of this subsection providing for the advancement of time, but only if that law provides that the entire State (including all political subdivisions thereof) shall observe the standard time otherwise applicable during that period, and (2) any State with parts thereof in more than one time zone may by law exempt either the entire State as provided in (1) or may exempt the entire area of the State lying within any time zone.
Arizona Revised Statute 1-242 makes that exemption for the state:
1-242. Standard time
A. The standard time in Arizona shall be the solar time of the one hundred fifth meridian west of Greenwich, commonly known as standard mountain time.
B. This section shall not be construed to affect the standard time established by United States law governing the movements of common carriers engaged in interstate commerce or the time for performance of an act by an officer or department of the United States, as established by a statute, lawful order, rule or regulation of the United States or an agency thereof.
C. Notwithstanding any other provision of law to the contrary by the United States government relating to adoption of daylight saving time by all of the states, the state of Arizona elects to reject such time and elects to continue in force the terms of subsection A, relating to standard time in Arizona.
D. The rejection of daylight saving time as provided for in this section may be changed by future legislative action.