Multi-Time Zone Split Counties in the United States
A Statutory Review under Title 49 CFR Part 71 & Census Cartographic Boundaries
Many consumer software applications erroneously assume that counties are atomic units of time zone jurisdiction. In this study, we profile five U.S. counties that are split by federal time zone boundaries, address a common misconception about El Paso County, TX, and summarize the statutory boundary lines. Other split counties exist, so treat this as a sample rather than a complete inventory.
1. Statutory Background: Uniform Time Act & 49 CFR Part 71
Under the Uniform Time Act of 1966 (15 U.S.C. §§ 260–267), authority over standard time zone boundaries rests exclusively with the United States Secretary of Transportation. The official boundaries are codified in Title 49 of the Code of Federal Regulations, Part 71 (49 CFR Part 71).
While the majority of time zone boundaries align along state lines or whole county perimeters, the DOT periodically establishes boundaries along natural physical features (such as rivers or mountain ranges) or survey meridians when doing so serves "the convenience of commerce."
2. The Five Legally Split Counties
| County / State | FIPS | Zones | Citation | Partition Geometry |
|---|---|---|---|---|
| Cherry County, Nebraska | 31031 | Central / Mountain | 49 CFR § 71.7(b) | Divided along the 101st meridian of west longitude. The eastern portion observes Central Time; the western portion observes Mountain Time. |
| Malheur County, Oregon | 41045 | Mountain / Pacific | 49 CFR § 71.9(b) | Most of the county, including Ontario and Vale, observes Mountain Time to synchronize with the Boise, Idaho metro area. The southern strip south of latitude 42°45′ N observes Pacific Time. |
| Idaho County, Idaho | 16049 | Mountain / Pacific | 49 CFR § 71.9(a) | The Salmon River bisects the county. Areas north of the Salmon River observe Pacific Time; areas south of the Salmon River observe Mountain Time. |
| Gulf County, Florida | 12045 | Eastern / Central | 49 CFR § 71.5(a) | The Intracoastal Waterway and Gulf county boundary partition: the northern inland portion around Wewahitchka observes Central Time; the southern coastal portion around Port St. Joe observes Eastern Time. |
| Culberson County, Texas | 48109 | Central / Mountain | 49 CFR § 71.7 | Officially entirely Central Time: 49 CFR § 71.7 places only El Paso and Hudspeth counties in the Mountain zone. Van Horn keeps Central Time, but the Guadalupe Mountains National Park area in the north informally keeps Mountain Time, so clocks there can differ by an hour. |
3. Correcting the Record: Why El Paso County is NOT Split
Multiple competitor mapping portals assert that El Paso County, Texas is divided between Central and Mountain Time. This claim is demonstrably false under federal law.
Under 49 CFR § 71.7, El Paso County is designated entirely within the Mountain Time Zone. The error likely stems from confusion with neighboring Hudspeth County (also entirely Mountain Time) or with Culberson County, which is officially Central Time but where the Guadalupe Mountains National Park area informally keeps Mountain Time. Open Map Lab checks county and time-zone boundaries against Census boundary polygons rather than repeating web scrape errors (boundary files are generalized, so borderline cases should be confirmed against 49 CFR Part 71).