The Day of Two Noons: November 18, 1883
Denver ran on Denver time. Salt Lake City ran on Salt Lake City time, about 13 minutes later. El Paso ran about 14 minutes earlier. Every city set its own clocks by the sun.
That worked fine for farmers. It was a disaster for railroads.
A train traveling from Chicago to San Francisco crossed dozens of local times. Schedules were printed in each town's own time. Passengers missed connections. Station masters kept conversion tables. The system was chaos.
On November 18, 1883, the railroads fixed it. They divided North America into four time zones: Eastern, Central, Mountain, and Pacific. At exactly noon, each railroad telegraph office switched to the new time. In cities that had been minutes ahead or behind, clocks stopped, then restarted. Many towns saw two noons that day: one for the old local time, one for the new railroad time. That is why it is called the "Day of Two Noons."
The railroads chose the 105th meridian west of Greenwich as the center of the Mountain Time Zone. That line runs through Denver, Salt Lake City, and straight down the Rocky Mountain spine. The zone was defined as exactly 7 hours behind Greenwich Mean Time.
Railroads and the Creation of US Time Zones
The railroad time zones were not law. They were an agreement among the major railway companies. Most cities adopted them voluntarily because it made sense. By 1895, about 80% of US towns used railroad time.
But there was no legal authority. A town could switch back. A state could ignore the whole system. Congress did nothing for 35 years.
The system held because the railroads needed it to hold. They controlled the telegraph lines that distributed time signals. They printed the schedules. If your town did not use railroad time, your train connections did not appear in the timetable.
The Standard Time Act of 1918 and DST Introduction
On March 19, 1918, Congress passed the Standard Time Act. It did two things.
First, it made the railroad time zones official US law. The act divided the country into five zones and gave the Interstate Commerce Commission authority to define zone boundaries.
Second, it introduced daylight saving time. The US was fighting World War I. DST was sold as a fuel-saving measure. Clocks went forward on March 31, 1918, and back on October 27.
The public hated it. Farmers complained that their schedules were tied to the sun, not the clock. Parents worried about children going to school in the dark. Congress repealed DST in 1919 but kept the time zones.
DST came back during World War II, year-round from February 1942 to September 1945. After that, it became a local option. States and cities could choose whether to observe it. Some did, some did not. Some observed it for part of the year. The result was chaos.
The Uniform Time Act of 1966
By the early 1960s, time in America was a mess. A bus traveling from West Virginia to Ohio could cross three different DST policies. Broadcast networks had to publish separate schedules for each market. The transportation industry begged for uniformity.
Congress passed the Uniform Time Act on April 13, 1966. It set uniform DST rules across the country: clocks forward on the last Sunday of April, back on the last Sunday of October. It also let states opt out entirely, but only by passing a state law exempting the entire state. Individual cities could not pick and choose.
The act also moved time zone authority from the Interstate Commerce Commission to the Department of Transportation, where it remains today.
The 105th Meridian: Why Mountain Time Is Where It Is
The Mountain Time Zone roughly follows the 105th meridian west of Greenwich. But "roughly" does a lot of work here.
The meridian itself runs through the middle of the zone. In theory, the zone extends 7.5 degrees east and 7.5 degrees west of that line. That would put the eastern boundary at 97.5 degrees west and the western boundary at 112.5 degrees west.
In practice, the boundaries follow state lines, county lines, and economic connections. Western Kansas and western Nebraska are on Mountain Time even though the 105th meridian is far to their west. The Idaho panhandle is on Pacific Time even though it is east of the meridian. These decisions were made by the Department of Transportation based on where people actually live and work.
The result is that the Mountain Time Zone is a narrow band through the Rockies that bulges eastward in the Great Plains. It includes the entire states of Colorado, New Mexico, Utah, and Wyoming, plus parts of Arizona, Idaho, Kansas, Montana, Nebraska, Nevada, North Dakota, Oregon, South Dakota, and Texas.
The Meridian and the Sun
The 105th meridian is not just a line on a map. It is the reference for solar noon in the zone. When the sun is directly over the 105th meridian, it is solar noon for the entire zone. Places east of the meridian see solar noon earlier. Places west see it later.
Boundary Oddities
Western Kansas has four counties on Mountain Time: Sherman, Wallace, Greeley, and Hamilton. The rest of the state is on Central Time. In Oregon, the northern portion of Malheur County observes Mountain Time while the rest of the state is on Pacific. West Wendover, Nevada, uses Mountain Time to align with Wendover, Utah. Idaho splits: the northern panhandle is on Pacific, the south on Mountain.
Why Boundaries Zigzag
The Department of Transportation can move a county from one zone to another if it serves the "convenience of commerce." A town that does most of its business with a city in the next zone over can petition to switch. These petitions are slow and contentious. The last major boundary change in the Mountain Time Zone was decades ago.
Why Does Arizona Stay on Standard Time Year-Round?
Arizona opted out of DST in 1968, the year after the Uniform Time Act took effect. The state legislature passed a law keeping Arizona on Mountain Standard Time year-round.
The reason was simple: summer heat. In Phoenix, an extra hour of evening daylight means an extra hour of air conditioning. Energy savings from DST vanish when people run AC from 5 PM to 7 PM. The legislature decided that staying on standard time was the smarter move.
The result is permanent confusion. From March to November, Arizona is on the same clock as Pacific Daylight Time, but it is not on Pacific Time. It is on Mountain Standard Time, which happens to match Pacific Daylight Time's offset. From November to March, Arizona matches the rest of the Mountain Time Zone.
The Navajo Nation Exception
There is an exception within the exception. The Navajo Nation, which covers northeastern Arizona, does observe DST. The Hopi Reservation, surrounded by the Navajo Nation, does not. This creates a DST "doughnut": a small area where clocks change, inside a larger area where they do not, inside a state where they do not. If you drive from Flagstaff to the Four Corners, you cross the DST boundary three times.
Scheduling with Arizona in Summer
The most common mistake: an East Coast person schedules a 2 PM "Mountain Time" call with Phoenix in July. Phoenix is on UTC-7 in July while Denver is on UTC-6. The call time is wrong by an hour. When scheduling with Arizona in summer, specify "Arizona time" or "MST." Avoid "Mountain Time" entirely.
Phoenix Daylight by Season
Phoenix on the March equinox (20 March 2026): sunrise 06:33, sunset 18:39, 12 hours 6 minutes of daylight. On the June solstice (21 June 2026): sunrise 05:18, sunset 19:40, 14 hours 22 minutes of daylight. On the September equinox (22 September 2026): sunrise 06:15, sunset 18:26, 12 hours 11 minutes of daylight. On the December solstice (21 December 2026): sunrise 07:28, sunset 17:24, 9 hours 56 minutes of daylight. No DST shift changes these times. The sun rises and sets on the same clock year-round.
Why Does Ciudad Juárez Still Change Clocks?
Mexico abolished DST nationally in October 2022. The law took effect in 2023. Most of Mexico now stays on standard time year-round.
But there is a border exception. Municipalities that share a border with the US can keep DST if they need to stay aligned with their US neighbors. Ciudad Juárez, across the Rio Grande from El Paso, Texas, chose to keep DST. It follows the same DST rules as the US Mountain Time Zone: clocks forward on the second Sunday of March, back on the first Sunday of November.
This created a new IANA time zone: America/Ciudad_Juarez. Before 2022, Ciudad Juárez was in the same zone as the rest of northern Mexico. Now it has its own zone, specifically to maintain alignment with El Paso.
Cross-border workers in Juárez and El Paso now have to check: does Mexico observe DST this year? For Juárez, the answer is yes. For the rest of Mexico, the answer is no.
The Border Economy
El Paso and Ciudad Juárez form a single economic region. Thousands of people cross the border daily for work, school, and shopping. A one-hour time difference between the two cities would create chaos at border crossings, schools, and workplaces. The Mexican government carved out the exception to prevent that.
Other Border Cities
Ciudad Juárez is not the only border city with an exception. Other northern municipalities also kept DST. But Juárez is the largest in the Mountain Time corridor. Its population of 1,512,450 makes it the second-largest city in the zone after Phoenix.
The IANA Split
Before the 2022 law, several northern Mexican cities shared a single IANA zone. The law forced a split. America/Ciudad_Juarez now exists as a distinct zone with its own DST rules, separate from America/Ojinaga and other Mexican zones. The IANA time zone database was updated within days of the law taking effect.
IANA and the Digital Era: America/Denver
Today, the Mountain Time Zone is represented in the IANA time zone database by the identifier America/Denver. This is the master record: it defines when DST starts and ends, what the UTC offset is at any given time, and what abbreviations to use.
The database also includes America/Boise, America/Cambridge_Bay, America/Ciudad_Juarez, America/Dawson_Creek, America/Edmonton, America/Fort_Nelson, America/Inuvik, and America/Phoenix. Each of these follows Mountain Time rules except America/Phoenix, which stays on standard time year-round.
When your phone or computer asks what time zone you are in, it is consulting this database. When you schedule a meeting in Denver for 2 PM Mountain Time, the system looks up America/Denver, checks whether DST is active, and converts accordingly.
How the Database Works
The IANA database is a set of text files maintained by volunteers. Each zone entry contains the UTC offset, DST transition rules, and historical changes going back decades. When Mexico changed its DST law in 2022, the database was updated within days. When a US county petitions the Department of Transportation to switch zones, the change appears in the next release.
MST Ambiguity in Code
MST can mean Mountain Standard Time (UTC-7), Malaysian Standard Time (UTC+8), or historically Moscow Standard Time (UTC+3). In time zone abbreviation databases, MST almost always means Mountain Standard Time, but context matters. Some systems display "MST" year-round even during MDT. This is technically incorrect but common.
Developers should use IANA zone identifiers like America/Denver rather than abbreviations. ISO 8601 offset notation (-07:00 or -06:00) avoids ambiguity entirely.
The 2007 DST Extension
The US Energy Policy Act of 2005 extended DST, effective 2007. Spring forward moved from the first Sunday in April to the second Sunday in March. Fall back moved from the last Sunday in October to the first Sunday in November. The IANA database reflects this change for every affected US and Canadian zone. Systems that were not updated after 2007 will show the wrong time during the extended DST weeks.
Quick Reference: Mountain Time Zone History
| Event | Date | What Changed |
|---|---|---|
| Railroads create time zones | 18 November 1883 | Four zones established; Mountain Time = UTC-7 |
| Standard Time Act | 19 March 1918 | Zones become US law; DST introduced |
| Uniform Time Act | 13 April 1966 | Uniform DST rules; states can opt out |
| Arizona exempts itself | 1968 | Arizona stays on MST year-round |
| US DST extended | 2007 | Spring forward moved to March; fall back to November |
| Mexico abolishes DST | October 2022 | Most of Mexico stops changing clocks |
| Ciudad Juárez keeps DST | 2023 | Aligns with El Paso; new IANA zone created |
For more details on specific topics: MST vs MDT: What's the Difference?, Why Doesn't Arizona Observe DST?, Mountain Time DST Dates, States in the Mountain Time Zone, and the America/Denver IANA zone.