How US time zone boundaries are set

US Time Zone Map · Updated October 31, 2026

The US Department of Transportation (DOT) decides where time zone lines run. The lines are written into federal regulations (49 CFR Part 71) and mostly follow state and county borders. A county can ask DOT to move it to a neighbouring zone; states can opt out of daylight saving time but cannot pick their zone on their own.

A short history

• 1883: US and Canadian railroads adopt standard time zones on November 18, replacing hundreds of local “sun times”. It was an industry agreement, not law.
• 1918: the Standard Time Act makes the zones federal law and adds daylight saving time. The Interstate Commerce Commission draws the boundaries.
• 1966: the Uniform Time Act standardises daylight saving dates; from 1967 the newly created DOT takes over the boundaries.
• 2005–2007: the Energy Policy Act moves DST to its current dates (second Sunday in March to first Sunday in November), effective 2007.

Who can change a boundary, and how

The legal test is the “convenience of commerce”: where people work, shop, commute and watch broadcasts. The usual path is a request from a county’s governing body (or a state on its behalf) to DOT, followed by a public notice, a comment period, sometimes a local hearing, and a final rule. Changes are rare and usually involve a county on a zone edge whose economy leans toward a city in the other zone.

Indiana is the best-known recent example. The state began observing daylight saving time statewide in 2006, and in the same period several counties petitioned to move between Eastern and Central time; some moved and a few later moved back. That is why the tz database has five separate Indiana zone ids. The Indiana time zone map shows the current split.

Why no two time zone maps agree

In 2022, an audit by DOT’s Office of Inspector General found that the department did not have an accurate, authoritative map of US time zone boundaries; DOT said it would develop one. Until an official map exists, every published map, including this one, is reconstructed from other sources. They differ for a few reasons:

• Split counties. A handful of boundaries run through a county rather than along its border, following a river, a range line or local custom.
• Local practice vs. the law. Some communities keep the time of a neighbouring city across the line. Phenix City, Alabama, informally uses Eastern time with Columbus, Georgia; West Wendover, Nevada, uses Mountain time with Utah.
• Tribal lands. The Navajo Nation observes daylight saving time while the rest of Arizona does not, and the Hopi Reservation inside it does not.
• Simplification. Small-scale maps often colour whole states, which hides the splits in 14 states.

How this site draws its map

The zone colours follow each state’s majority zone, and the 14 split states are marked and explained on their own pages. Live times come from the IANA time zone database built into your browser, which is maintained by volunteers who track every boundary and DST change worldwide. That database answers “what time is it in this zone id”, which is exact; it does not answer “which zone is this street in”, which is the part that varies between maps. The sources are listed on the data sources page.

Key takeaways

• DOT sets the lines (49 CFR Part 71); they mostly follow county borders.
• Counties can petition to move; the test is the convenience of commerce.
• No official federal map existed as of the 2022 audit, so maps differ in the details.
• For a specific address near a line, check with the local county.

Explore it: the live US time zone map, the state time zone maps, and the abbreviations guide.

Sources

• 15 U.S.C. §§ 260–267 (Standard Time Act and Uniform Time Act); 49 CFR Part 71 (standard time zone boundaries).
• US DOT Office of Inspector General, 2022 audit of DOT’s oversight of time zones, as reported by ABC7.
• IANA time zone database (tz), via the browser’s Intl API.