Where is the timezone defined for Arizona? Is it in the Arizona constitution? Is it in Arizona Revised Statutes? Is it in the United States constitution? Is it in the United States Code? Where is Arizona's timezones legally defined?

What about other states? What about other non-states, like Washington D.C., and reservations? Are there any laws governing timezones federally? Is every state legally required to establish a timezone? It the timezone based on UTC, GMT, or any other time standard? What is the history of legal timezone definitions? Do Indian reservations have to define a timezone?

EDIT: If federal law governs timezones, is it within their constitutional powers to do so, or could libertarians (or other party) argue otherwise? Could a state use their 10th ammendment rights (along with any other state rights) to override any federal law or standard?

  • An interesting bit of history is that when time zones were initiated (due to railroad time tables) there were people who protested that it was god-given than noon was when the sun was overhead and the government could not say it was not noon. May 21 '20 at 16:31
  • The US constitution is short enough to read fairly quickly. Doing so will make it clear that time zones are established elsewhere.
    – phoog
    May 22 '20 at 4:02

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.

  • Arizona was keeping itself on standard time long before 2005.
    – phoog
    May 22 '20 at 4:04
  • @phoog - Here's a good history of Arizona's time zone evolution: timeanddate.com/time/us/arizona-no-dst.html. Arizona tried DST in 1967 and didn't like it. I think, but don't know for sure, that it was up to local jurisdictions on whether or not to observe DST until the 2005 change that required DST observation to be set at the state level, though Arizona may have had it specified at the state level long before local specification was prohibited. Either way, it is now a state-wide standard for Arizona except for the Navajo Nation.
    – Dave D
    May 22 '20 at 4:46
  • If you want to call the Navajo Nation a "local jurisdiction," that's true, but they're not really subject to the jurisdiction of the state if I understand correctly. As far as I can tell that has been the only portion of Arizona that has employed DST since at least the early 1990s, and they continue to do so since 2005. Since parts of the Navajo Nation are in other Mountain Time states, they can't be internally consistent and agree with all of the states they overlap with; they have to diverge either from Arizona or from the other states.
    – phoog
    May 22 '20 at 8:52

The basis for timezones within the US is part of the 15 U.S. Code SUBCHAPTER IX— STANDARD TIME, which defines the offset from UTC, names and special cases. The actual specific boundaries are formed by the basis of an order from the Transportation Secretary.


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