Before the establishment of time zones in 1883, there were more than 144 local times in North America.¹ The resulting time differences between adjacent towns and cities were not critical when it took days to travel from place to place.² With the proliferation of railroads, faster travel became possible across large geographies, and travelers could sometimes arrive at an earlier local time than they had departed.² Due to this lack of time standardization, train scheduling proved difficult to coordinate, resulting in missed connections and collisions.¹ As a result, the major railroad companies began to operate on a coordinated system of four time zones starting in 1883.²

Because the development of standardized time was transportation-driven, the government coordination of time zones was handled by transportation agencies.³ In 1918, the federal organization in charge of railroad regulation — the Interstate Commerce Commission (ICC) — was given the power to address coordination concerns.^{4 3} That year, five time zones were officially adopted as the US entered World War I: the Eastern, Central, Mountain, Pacific, and Alaska zones, all of which are still in use today.⁴ However, the need for coordination among all transportation modes became increasingly important after World War II.⁵ When the Department of Transportation was created by Congress in 1966, it was assigned “the responsibility of regulating, fostering, and promoting widespread and uniform adoption and observance of standardized time” within each time zone.^{4 5 6}

Daylight Saving Time (DST) was enacted as a legal requirement by the Uniform Time Act of 1966.^{6 7} Motivated by transportation improvements, this act mandated standard time within the existing time zones and established a permanent system of uniform DST, including the dates and times for twice yearly transitions. ⁶ While State governments cannot independently change time zones or the length of DST, they can exempt themselves from DST, independent of DOT authority or permission. ⁶ Nonetheless, DST is observed uniformly across the nation except in American Samoa, Guam, Northern Mariana Islands, Puerto Rico, the Virgin Islands, Hawaii, and most of Arizona.⁸

Today, the Department of Transportation continues to oversee standard time due to its historical and contemporary importance in transportation and associated commercial activity.⁷ Time zone boundaries, established by law, can only be changed by the Secretary of Transportation upon a determination that the proposed adjustment serves the “convenience of commerce.” ⁹ Per DOT policy, a petition requesting such a change must come from the highest political authorities in a State or locality. Several communities have requested changes to their time zone designation over the past two decades, the most recent being Mercer County, North Dakota in 2010, which chose to switch from Mountain to Central Time.¹⁰ Authorizing these changes and keeping track of the legally designated time zone for each area of the U.S. are key facets of the DOT’s oversight of uniform time observance, time zones, and DST. ⁶

In 2019, the Bureau of Transportation Statistics (BTS), in coordination with the Office of the General Counsel, created a digital geographic representation of the official written time zone delineations defined in the Code of Federal Regulations (CFR), Title 49, Subtitle A, Part 71 - Standard Time Zone Boundaries. Currently the United States and its territories have 9 time zone boundaries: Atlantic, Eastern, Central, Mountain, Pacific, Alaska, Hawaii–Aleutian, Samoa, and Chamorro.

The DOT Time Zone Boundary Geospatial layer is the verified digital representation of the current time zone delineations as written in the CFR, and is part of the National Transportation Atlas Database (NTAD). This layer provides the American public with detailed, reliable, and authoritative information on time-related authorities and time zone boundaries.

Below is a map utilizing this verified geospatial layer showing the current time zone delineations. The CFR delineates the official boundaries using text that describes coordinates and geographic features such as—but not limited to—lines of longitude, State or county lines (administrative boundaries), and rivers. ⁶ An interactive web mapping application containing the time zone boundaries is also available.

SOURCES:

1          Phillips, Charles. A Day To Remember: November 18, 1883. American History, Dec. 2004, pp. 16-18.

2          Gordon, John Steele. Standard Time: We All Live By What Happened on November 18, 1883. American Heritage, Jul./Aug. 2001, pp. 22-23. Available at https://www.americanheritage.com/standard-time as of December 2022.

3          United States. 1966. Department of transportation act: report to accompany H.R. 15963. Washington, DC: U.S. GPO. Available at https://catalog.hathitrust.org/Record/007833545 as of December 2022.

4          Clark, Corrie E., and Lynn J. Cunningham. 2020. Daylight Saving Time (DST). CRS Reports (Library of Congress. Congressional Research Service). Available at https://crsreports.congress.gov/product/pdf/R/R45208/8 as of December 2022.

5          United States. 1966. Creating a Department of Transportation: Hearings before a Subcommittee of the Committee on Government Operations, House of Representatives, 89th Congress, 2nd session, on H.R. 13200 a bill to establish a Department of Transportation and for other purposes. Washington, DC: U.S. GPO.

6          U.S. Department of Transportation. DOT Can Improve Processes for Evaluating the Impact of Time Zone Changes and Promoting Uniform Time Observance (9/20/2022). Available at https://www.oig.dot.gov/library-item/39090 as of December 2022.

7          U.S. Department of Transportation. Uniform Time (9/29/2022). Available at https://www.transportation.gov/regulations/time-act as of December 2022.

8          U.S. Department of Transportation. Daylight Saving Time (7/25/2022). Available at  https://www.transportation.gov/regulations/daylight-saving-time as of December 2022.

9          U.S. Department of Transportation. Procedure for Moving an Area from One Time Zone to Another (9/29/2022). Available at https://www.transportation.gov/regulations/procedure-moving-area-one-time-zone-another as of December 2022.

10        U.S. Department of Transportation. Recent Time Zone Proceedings (6/06/2012). Available at https://www.transportation.gov/regulations/recent-time-zone-proceedings as of December 2022.

11        National Hurricane Center and Central Pacific Hurricane Center, National Oceanic and Atmospheric Administration. What is UTC or GMT Time? Available at https://www.nhc.noaa.gov/aboututc.shtml as of December 2022