History of Time Zones and Daylight Saving Time (DST) (2024)

Before the establishment of time zones in 1883, there were more than 144 local times in North America.¹The resulting time differences between adjacent towns and cities were not critical when it took days to travel from place to place.²With 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.²Due to this lack of time standardization, train scheduling proved difficult to coordinate, resulting in missed connections and collisions.¹As 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.³In 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.⁴However, the need for coordination among all transportation modes became increasingly important after World War II.⁵When 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.⁷Time 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.”⁹Per 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.¹⁰Authorizing 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.

Time Zone	Relationship to Coordinated Universal Time (UTC)¹¹
Atlantic	UTC minus 4 hours
Eastern	UTC minus 5 hours
Central	UTC minus 6 hours
Mountain	UTC minus 7 hours
Pacific	UTC minus 8 hours
Alaska	UTC minus 9 hours
Hawaii–Aleutian	UTC minus 10 hours
Samoa	UTC minus 11 hours
Chamorro	UTC plus 10 hours