Mountain Daylight Time (MDT)
UTC offset: -06:00
Standard offset: -07:00 (MST)
IANA identifier: America/Denver
Abbreviation: MDT
Population affected: approximately 25 million (US and Canada)
DST period: Second Sunday in March to first Sunday in November
Mountain Daylight Time advances the Mountain Time Zone one hour from MST (UTC-07:00) to UTC-06:00. The zone covers the Rocky Mountain corridor of the United States and Canada, from Montana to New Mexico, from Colorado to parts of Idaho and Oregon.
The notable exception: Arizona (except the Navajo Nation) does not observe DST and stays on MST year-round. This creates seasonal confusion along the Arizona border with neighboring states.
Denver
The "Mile-High City" (~715,000, metro ~2.9 million) at exactly 5,280 feet elevation. State capital of Colorado, a booming tech hub, and the gateway to Rocky Mountain skiing. Denver's economy has diversified from mining and ranching into aerospace, tech, healthcare, and cannabis (Colorado legalized recreational marijuana in 2012).
The city's location (central US north-south, mountain west) makes it a major airline hub (Denver International Airport is the third-busiest in the US). MDT places Denver one hour behind Eastern cities and one hour ahead of Pacific cities, a middle position that's convenient for national coordination.
Salt Lake City
Utah's capital (~200,000, metro ~1.3 million). Headquarters of the Church of Jesus Christ of Latter-day Saints (LDS/Mormon church), which shapes the city's culture profoundly. Also a growing tech sector ("Silicon Slopes"), outdoor recreation hub (skiing at Park City, Alta, Snowbird within 45 minutes), and host of the 2002 Winter Olympics (with a 2034 bid confirmed).
Calgary
Alberta's largest city (~1.4 million), Canada's energy capital (oil and gas headquarters cluster here). The Calgary Stampede (July) is billed as "The Greatest Outdoor Show on Earth," a 10-day rodeo and exhibition drawing over a million visitors. Calgary's economy booms and busts with oil prices.
The Arizona Exception
Arizona (population ~7.4 million) does not observe DST. The reasoning: summer temperatures regularly exceed 40°C (105°F). Extending evening daylight would extend the hottest part of the day, increasing air conditioning costs rather than saving energy. The Navajo Nation (which extends into Arizona, Utah, and New Mexico) does observe DST, creating pockets within Arizona that change clocks.
This means in summer: driving from California (PDT, -07:00) into Arizona (MST, -07:00) involves no time change. But driving from Arizona into New Mexico (MDT, -06:00) loses an hour. In winter, the opposite: Arizona and Mountain Standard are the same (-07:00).
Other Major MDT Cities
Albuquerque (~560,000): New Mexico's largest city, home to Sandia National Laboratories and the International Balloon Fiesta.
Boise (~230,000): Idaho's capital, a fast-growing tech and outdoor recreation center.
El Paso (~680,000): Texas's westernmost city, on Mountain Time (not Central), twinned with Ciudad Juarez across the border.
Edmonton (~1 million): Alberta's capital, a government and university city.
Scheduling
At UTC-06:00 (MDT):
- Eastern (EDT): 2 hours ahead
- Central (CDT): 1 hour ahead
- Pacific (PDT): 1 hour behind
- Arizona (MST): 1 hour behind (summer only)
- UK (BST): 7 hours ahead
Neighboring Zones
| Zone | Offset (summer) | Difference from MDT |
|---|---|---|
| Pacific (PDT) | UTC-07:00 | 1 hour behind |
| Central (CDT) | UTC-05:00 | 1 hour ahead |
| Arizona (MST) | UTC-07:00 | 1 hour behind |
| Eastern (EDT) | UTC-04:00 | 2 hours ahead |
Technical Identifiers
- America/Denver (IANA canonical US)
- America/Edmonton (IANA canonical Canada)
- MDT (Mountain Daylight Time)
- MST (winter: Mountain Standard Time, UTC-07:00)
- Windows: "Mountain Standard Time"
- DST rule: US/Canada schedule (2nd Sunday March, 1st Sunday November)
- Military/aviation: S ("Sierra") for UTC-06:00
Quick Reference
| Attribute | Value |
|---|---|
| UTC offset (summer) | -06:00 |
| UTC offset (winter) | -07:00 |
| DST observed | Yes (except Arizona) |
| IANA zone | America/Denver |
| Population | ~25 million |
| Key cities | Denver, Salt Lake City, Calgary, Phoenix (no DST) |
| Arizona exception | No DST (too hot) |
| Navajo Nation | Observes DST (within Arizona) |
| Major airport | Denver International (3rd busiest US) |