Arizona does not observe Daylight Saving Time (DST) from March through November each year, so for half the year, the time in Phoenix, Flagstaff, and other cities in Arizona will be different than other places in the Mountain Standard Time (MST) zone. Put another way, from March through November during DST, the time in Arizona is the same as that of California's Pacific Daylight Time (PDT) zone.
Mountain Standard Time is seven hours behind the Universal Time, Coordinated (UTC) during Standard Time and eight behind during DST, but Phoenix remains seven hours behind because UTC doesn't adjust for Daylight Saving Time. Other states included in the MST zone are Colorado, Montana, New Mexico, Utah, Wyoming, and parts of Idaho, Kansas, Nebraska, North Dakota, Oregon, South Dakota, and Texas.
Whether you're visiting Phoenix or Flagstaff, knowing how you'll need to reset your watch when you arrive in Arizona will help you stay on time during your trip. However, keep in mind that if you're visiting the southern Navajo Nation, it does observe Daylight Savings Time.
Why Arizona Doesn't Observe DST
Although Daylight Saving Time was established by federal law in 1966 with the passage of the Uniform Time Act, a state or area may choose not to observe it. However, it must always observe DST at the same time as the rest of the United States if it does choose to observe this time change.
The Arizona State Legislature voted to not adhere to the new legislation in 1968 largely due to costs associated with cooling homes in the evenings after work. Since Arizona typically reaches triple-digit temperatures most of the summer, the resulting "extra hour of daylight" only served to increase costs of air conditioning since families would be spending more hours of sweltering daytime heat at home.
Although legislation has been introduced in Arizona several times in recent years to start adhering to Daylight Saving Time like the rest of the country, each time it's been met with outrage from local residents. Other areas in the U.S. that don't observe Daylight Saving Time are Hawaii, American Samoa, Guam, Puerto Rico, and the Virgin Islands—and until 2005, Indiana.
How to Know the Time in Arizona
Although cell phones and smartwatches have made manually updating the time on your devices almost obsolete when traveling, it can still be beneficial to know how to calculate the time in Arizona based on the Universal Time Coordinated.
UTC is a time standard based on Earth's rotation that, like Greenwich Mean Time, measures the solar time on the Prime Meridian (0 degrees longitude) in London, England. The UTC is the standard for how to set clocks and understand time around the world.
Since neither the state of Arizona nor the Universal Time, Coordinated observe Daylight Saving Time, Arizona is always UTC-7—seven hours behind the Universal Time. If you know what the UTC is, no matter what time of the year it is, you can always know you're just seven hours behind in Arizona.