How Time Zone Boundaries Work: Political Law vs Solar Longitude

In the United States, time zone boundaries are not set by the National Institute of Standards and Technology or by local governments. Under the Standard Time Act of 1918 and the Uniform Time Act of 1966 (15 U.S.C. §§ 260–267), the authority to set and move zone boundaries rests with the Secretary of Transportation.

The boundaries are written into Title 49, Part 71 of the Code of Federal Regulations. A county or state that wants to change zones asks the Department of Transportation, which considers whether the move serves "the convenience of commerce", including where people shop, work, travel and get their broadcast news.

The Uniform Time Act also governs daylight saving time (15 U.S.C. § 260a). States may opt out and stay on standard time all year, as Hawaii and most of Arizona do, but a state cannot adopt permanent daylight saving time without an act of Congress. The current dates, and where the Sunshine Protection Act stands, are covered in daylight saving time 2026.

If time zones followed meridians exactly, the boundary would cut through metropolitan areas, separating commuters from their offices and splitting broadcast markets. Instead, boundaries usually follow county or state lines and are drawn around the economic centers that communities depend on.

Indiana is a familiar example. The northwest corner near Chicago and the southwest corner around Evansville observe Central Time, while most of the state, including Indianapolis, observes Eastern Time. Similar jogs run through Kentucky, Tennessee, the Dakotas, Nebraska, Kansas, Texas and the Florida Panhandle.

Outside the U.S., national decisions dominate. China spans roughly the width of five geometric zones but uses one official time, and Spain uses Central European Time even though most of the country lies west of the Greenwich meridian. The world time zone map makes these political shapes easy to see.

Congress fixes the number of zones and their offsets: 15 U.S.C. § 261 divides the country into nine zones, from UTC−4 (Atlantic) to UTC+10 (Chamorro). The lines between them are prose descriptions in 49 CFR Part 71, one section per zone or boundary, written in the language of surveys and county records rather than coordinates:

Rivers, county lines and city limits are used because they are what residents and businesses already know. They are also why the lines look jagged on a map and why a few counties end up divided. For the resulting count of zones, a map of all six that cover the states, and the 14 states that straddle a line, see how many time zones are in the US.

In the U.S., daylight saving time currently begins on the second Sunday in March and ends on the first Sunday in November. The U.S. time zone map shows where each zone's boundary falls.

Software rarely stores times as "Eastern" or "EST". It uses IANA time zone identifiers such as America/New_York, America/Chicago or Europe/London. Each ID names a representative city and stands for a region whose clocks have agreed since 1970, including its full history of offset and DST changes.

That is why one U.S. zone can have several IDs. America/Phoenix and America/Denver are both Mountain Time, but only Denver observes DST. Indiana has several America/Indiana/... IDs because its counties changed their rules at different times. Using the right ID is what lets a calendar compute past and future offsets correctly.

Abbreviations are ambiguous: CST can mean U.S. Central Standard Time, China Standard Time or Cuba Standard Time. When sharing a meeting time, give either a UTC offset or a city. To find a meeting time across time zones, pick the two cities rather than their abbreviations and compare the hours they share.

The lookup has two parts:

1. Which zone are you in? Your coordinates are matched against time zone polygons derived from the timezone-boundary-builder project (via the tz-lookup library). The result is an IANA zone ID. These polygons are generalized to roughly 1 km precision near borders, so a location very close to a zone boundary can be assigned to the neighboring zone.
2. What time is it there? The current offset, DST status and zone names come from your browser's built-in internationalization data (the Intl API, backed by ICU). They are as current as your browser's copy of the time zone rules.

To use it, open What Time Zone Am I In: it shows your device time zone straight away, a GPS check matches your exact position, and the search looks up any place by name or ZIP code. Near a zone boundary, confirm with a local source, for example a county government website, before scheduling anything important.

Most scheduling errors come from a few predictable traps:

• Assuming DST changes on the same day everywhere. The U.S. and the European Union switch on different weekends, so for a few weeks each spring and autumn the gap between New York and London is one hour shorter than usual.
• Forgetting the southern hemisphere. Countries such as Australia and Chile observe DST during the northern winter, so their offset from you can change twice a year in opposite directions.
• Ignoring non-hour offsets. India uses UTC+5:30, Nepal UTC+5:45 and parts of Australia UTC+9:30.
• Crossing the date line. Some Pacific nations are 24 or more hours ahead of Hawaii, so "Tuesday afternoon" can already be Wednesday there.
• Using abbreviations instead of zone IDs. See above.

The time difference calculator compares two places on a specific date, which avoids most of these traps.