Current US Law
There are two areas of applicable federal law: the United States Code, which is changed by Congress, and the Code of Federal Regulations, which in this case is changed by the US Department of Transportation.
15 USC 260a
The United States Code which regulates Daylight Saving Time is found here. Of particular interest is the following paragraph:
(b) State laws superseded
It is hereby declared that it is the express intent of Congress by this section to supersede any and all laws of the States or political subdivisions thereof insofar as they may now or hereafter provide for advances in time or changeover dates different from those specified in this section.
The bottom line here is that a state (or other political subdivision — see the Wayne County, KY reference below) can only:
- Opt out of DST
- If not opted out of DST, you must advance an hour starting on the second Sunday of March and ending on the first Sunday in November.
- Petition the Secretary of the Department of Transportation and request a change of time zone designation.
The complete Subchapter IX entitled “Standard Time”, with regulations beyond just DST, can be found here. This House of Representatives site contains helpful historical references to changes made as far back as 1948.
49 CFR Part 71
The US Government Printing Office has an electronic version of the Code of Federal Regulations online. Title 49 is called “Transportation”, and generally updated by the Secretary of the Department of Transportation. Part 71 is called “Time Zone Boundaries”, and contains detailed descriptions of all nine US time zone boundaries and Daylight Saving Time, referred to in the CFR as “Annual advancement of standard time”. See the relevant CFR part here.
Changes From the Federal Register
US DOT amended 49 CFR Part 71 on March 13, 2013 to comply with acts of Congress in prior years. See notice in this Federal Register excerpt (starting third column of the first page, which is labeled page 15883).
Back in 2000, there was a request from a Wayne County in Kentucky to change time zone from the Central Time Zone to the Eastern Time Zone. It seems that this particular request was more difficult than most which are received by the US DOT, and the response we found in this Federal Register excerpt (starting bottom of the first column of the first page, which is labeled page 50154) describes most of the activity and the decision. Any jurisdiction — state or local — proposing to change its time zone would do well to read this entry.