Texas time zone map

The federal boundary between the Central and Mountain zones enters Texas at the New Mexico state line, turns south at the northeast corner of Hudspeth County, and follows Hudspeth's eastern county line all the way to the Rio Grande. That eastern line is shared first with Culberson County, then with Jeff Davis and Presidio. Everything west of it — Hudspeth and El Paso counties — is Mountain; everything east of it, including Culberson County and its seat at Van Horn, is Central.

On the ground the change happens on I-10 between Van Horn and Sierra Blanca, about 30 miles apart. Drive west out of Van Horn on Central Time and the clock goes back an hour before Sierra Blanca; drive east and you lose it again. There is no sign at a state line to warn you, and phones often switch late because the nearest cell towers may be in the other zone.

Culberson County is the ragged edge of the rule. The county is officially Central, but the northwestern corner of it, around Guadalupe Mountains National Park, sits far enough west that the park and the scattered ranches near it run on Mountain Time in practice — the National Park Service tells visitors to set their phones to Mountain Time before arriving. Pine Springs is roughly 55 road miles northwest of Van Horn on US-62/180, which puts it well past the meridian where the rest of the county's neighbors have already changed.

All of Texas observes daylight saving time, in both zones and on the same dates. Clocks go forward one hour at 2:00 a.m. on the second Sunday in March and back one hour at 2:00 a.m. on the first Sunday in November. Because both zones change together, El Paso stays exactly one hour behind Houston every day of the year.

The offsets: Central Standard Time is UTC−6 and Central Daylight Time is UTC−5; Mountain Standard Time is UTC−7 and Mountain Daylight Time is UTC−6. The Texas Legislature has repeatedly taken up bills to stop changing the clocks, but a state may not adopt permanent daylight time without a change in federal law — it may only opt out of DST entirely, which Texas has not done.

Three of the four states bordering Texas keep Central Time: Oklahoma, Arkansas and Louisiana. Only New Mexico is Mountain, which is why the clock changes when you cross the state line west of Amarillo on I-40 or US-60, and again west of Lubbock on US-84. The Panhandle stays Central even though it lies farther west than parts of Mountain-time New Mexico — the zone line there is the state boundary, not a meridian.

Across the Rio Grande the picture differs by city. Ciudad Juárez, opposite El Paso, keeps the same clock as El Paso, so the two halves of that metro never disagree. Farther down the river, the Mexican cities across from Laredo, McAllen and Brownsville are on Central Time with Texas, though Mexico's daylight-saving rules differ from the U.S. ones outside the border zone, so the two sides can be an hour apart for a few weeks in spring or autumn.

Scheduling anything in far West Texas is worth double-checking: a business in Van Horn and one 30 miles west in Sierra Blanca run an hour apart.