It’s called daylight saving time because back when modern time adjustment was first adopted, the idea was to, well, save daylight. Specifically, that means the evenings are longer in the summer, offering people more natural light for work and play, and theoretically, it means we use less energy if we don’t need to switch on the lights until later in the evening.

Clocks should be turned back an hour, and no doubt you’ve heard the mnemonic that when time changes in the autumn, Americans “fall back.”

(European Union nations switch a weekend before the U.S. schedule. The E.U. changed clocks on October 28.)

The U.S. has experimented with daylight saving time for nearly a century, dating back to 1918, when, in support of the war effort, it adopted what was then called “Fast Time” for a period of seven months before it was repealed. Something similar happened again during World War II, when, from February 1942 until September 1945, the entire country set the clocks ahead one hour and called the move “War Time.”

The current phase of daylight saving time in the U.S. dates back 50 years to the federal Uniform Time Act of 1968, which suggested states agree upon standard time zones. Because it didn’t mandate adhering to daylight sayings time, which is why there are still outliers.

Arizona is perhaps the most prominent of the remaining holdouts that refuses to fall in line with the rest of the nation and observe daylight saving time. The Grand Canyon State’s legislature opted out of the 1968 law for a pretty fair reason: Arizona doesn’t need more hours of blazing sunlight and daytime heat. And so, most of the state simply doesn’t observe the biannual time change.

The exception to the exception is within Native American nations in Arizona, which independently choose whether or not to observe. So while the state’s Navajo nation observes DST, the Hopi nation does not. This presents an interesting occurrence as the Arizona Navajo nation surrounds the Hopi, so anyone passing between tribal lands may experience time changes within a short geographic span.

Hawaii also does not observe daylight saving time, nor do U.S. territories American Samoa, Guam, or the U.S. Virgin Islands. U.S. commonwealths the Northern Mariana Islands and Puerto Rico also refrain from observing DST.