If you sell to, call, or travel in the United States, the timezone layer of that country is a minefield of political compromise. Six major zones, one state with two clocks, a territory that runs on a different standard, and a widely-repeated myth about DST that is true in Arizona and false almost everywhere; it's the easiest place in the world to be an hour wrong simply from trusting a map.
The Six Zones, Simply Put
| Zone | UTC (winter/summer) | Main regions |
|---|---|---|
| Eastern | UTC−5 / UTC−4 | New York, DC, Atlanta, Miami, Boston |
| Central | UTC−6 / UTC−5 | Chicago, Dallas, Houston, New Orleans |
| Mountain | UTC−7 / UTC−6 (Arizona: no DST) | Denver, Salt Lake City, plus half of Arizona |
| Pacific | UTC−8 / UTC−7 | Los Angeles, Seattle, San Francisco, Portland |
| Alaska | UTC−9 / UTC−8 | Most of Alaska (Aleutian Islands minus an hour) |
| Hawaii-Aleutian | UTC−10 (no DST) | Hawaii year-round; Aleutian Islands in winter |
Arizona: The Two-Clock State
Arizona famously doesn't observe DST. The desert decided the extra evening hour of heat wasn't a benefit, so from March to November it sits on UTC−7 while most of the rest of the country is on UTC−4 to UTC−6 elsewhere. But the catch that surprises everyone: the Navajo Nation, which covers a large part of northeastern Arizona, does observe DST. So within Arizona, for half the year, the state has two clocks depending on which part of it you're in. In winter, things line up; in summer, Phoenix (Mountain Standard, no DST) equals Mountain Daylight in Colorado at UTC−6.
Indiana: The Mess That Took Decades
Indiana was the textbook example of timezone chaos: parts of the state used Eastern, parts used Central, some of the Eastern parts observed DST and some didn't, and the state legislature spent the better part of the century relitigating it. The modern mess mostly resolved in 2006, when the whole state went on Eastern time with DST, though a small band of counties around the Illinois border still runs on Central. A truck farm on one side of a county line and a manufacturing plant on the other could be an hour apart, and that's not folklore, it's the current map.
The Territories Forgot to Match the Map
The territories quietly break the "6 zones" framing. Puerto Rico runs UTC−4 year-round, Eastern Standard Time without DST, which means in winter it matches New York and in summer it's an hour behind it. The US Virgin Islands share that. Guam runs UTC+10, American Samoa UTC−11, and the minor outlying islands scatter further. None of these territories observe US DST.
DST: When the Whole Country Moves
All the DST-observing US zones change together, on the second Sunday in March and first Sunday of November. That means: the difference between New York and Los Angeles is always 3 hours, all year. The same cannot be said for the US vs. Europe, which change on different dates; see the DST guide for why the two-week gap keeps happening.
Why This Matters to Everyone Else
If you're scheduling from outside the US, these quirks compound: call Atlanta at "their 9 AM" and you've assumed Eastern time; call Phoenix at "their 9 AM" and the DST season can flip the answer by two hours once the eastern city in your invite also changes. My rule for US work is brutal simplicity: convert by city name + date, never by "the US," and recheck in late March and late October. The date-aware converter has every zone above built in, including Phoenix and the Navajo exception, because the database treats them separately.