Phoenix, Arizona keeps its clocks on Mountain Standard Time all year. That single decision, made in 1968 to avoid extending summer air-conditioning loads into the evening, makes a city of 1,650,070 the largest permanent-MST population in the zone. Ciudad Juárez, Mexico (1,512,450) does the opposite: it observes DST specifically to stay synchronised with El Paso, Texas, even though Mexico abolished the practice nationally in October 2022.
Major US cities on Mountain Time
Denver, Colorado (729,019) is the canonical city for the zone. Full DST schedule: clocks spring forward the second Sunday of March and fall back the first Sunday of November.
Phoenix, Arizona (1,650,070) is the largest city in the zone and the most confusing. From March to November, Phoenix matches Pacific Daylight Time (UTC-7), but it is not on Pacific Time. It stays on Mountain Standard Time year-round.
Albuquerque, New Mexico (564,559), Colorado Springs (456,568), Aurora (359,407), and El Paso, Texas (678,815) all observe DST. El Paso sits on the far western edge of Texas and uses the same zone as Denver.
Other notable US cities: Boise, Idaho (southern Idaho is Mountain; northern Idaho is Pacific), Tucson (542,629), Mesa (471,825), Chandler (260,828), Gilbert (247,542), Glendale (240,126), and Scottsdale (236,839). All Arizona cities on this list are on permanent MST.
Canadian cities on Mountain Time
Calgary, Alberta (1,306,784) and Edmonton, Alberta (1,010,899) both observe DST on the same schedule as the US. Alberta held a 2021 referendum on permanent DST; it was rejected by 50.2% of voters. Check with Elections Alberta for the final certified result.
Dawson Creek and Fort Nelson in British Columbia stay on MST year-round (no DST), creating a pocket of permanent MST inside a province that otherwise uses Pacific Time.
Mexican cities on Mountain Time
Ciudad Juárez, Chihuahua (1,512,450) is the only major Mexican city on Mountain Time that observes DST. Mexico abolished DST nationally in October 2022, but northern border municipalities were exempted. Juárez keeps DST to match El Paso's clock.
Other Chihuahua cities on Mountain Time do not observe DST. In winter they match MST (UTC-7); in summer they stay on UTC-7 while the US switches to MDT (UTC-6), so they fall one hour behind El Paso and Juárez during summer.
Cities in split states
Several US states straddle the Mountain, Pacific or Mountain, Central boundary.
- Idaho: Southern Idaho (Boise) is Mountain; northern Idaho (Coeur d'Alene) is Pacific.
- Oregon: Malheur County, in the southeast corner, is Mountain. The rest of Oregon is Pacific.
- Nevada: West Wendover uses Mountain Time to stay aligned with Wendover, Utah. The rest of Nevada is Pacific.
- Kansas: Four western counties (Sherman, Wallace, Greeley, Hamilton) are Mountain. The rest of Kansas is Central.
For the full list of states in the zone, see: Which States Are in the Mountain Time Zone?
Reference table: largest cities on Mountain Time
| City | Country | Population | IANA Zone | DST? |
|---|---|---|---|---|
| Phoenix | US | 1,650,070 | America/Phoenix | No |
| Ciudad Juárez | MX | 1,512,450 | America/Ciudad_Juarez | Yes |
| Calgary | CA | 1,306,784 | America/Edmonton | Yes |
| Edmonton | CA | 1,010,899 | America/Edmonton | Yes |
| Denver | US | 729,019 | America/Denver | Yes |
| El Paso | US | 678,815 | America/Denver | Yes |
| Albuquerque | US | 564,559 | America/Denver | Yes |
| Tucson | US | 542,629 | America/Phoenix | No |
| Mesa | US | 471,825 | America/Phoenix | No |
| Colorado Springs | US | 456,568 | America/Denver | Yes |
Quick rule of thumb: Any city in Arizona is on MST all year. Any city in Colorado, New Mexico, Utah, Wyoming, Montana, or Alberta observes DST. Ciudad Juárez observes DST; the rest of Mexico's Mountain Time cities do not.