Twice a year most Americans change their clocks. The time between those changes is daylight saving time.
Key idea
In most states, clocks move ahead one hour at 2:00 a.m. on the second Sunday in March and back one hour on the first Sunday in November: "spring forward, fall back." Hawaii and most of Arizona stay on standard time all year (the Navajo Nation in northeastern Arizona does change).
Why it works
Why move the clocks? In summer the Sun rises very early, while many people are asleep. Moving clocks ahead shifts an hour of daylight from the early morning to the evening, when more people are awake to use it. Near the tropics, like in Hawaii, day length changes little through the year, so there is little to gain. Arizona is hot in summer, and an extra hour of evening sun is not welcome there.
| City | Winter (standard time) | Summer (daylight saving time) |
|---|---|---|
| Los Angeles | 8 hours behind UTC | 7 hours behind UTC |
| Phoenix | 7 hours behind UTC | 7 hours behind UTC |
| Denver | 7 hours behind UTC | 6 hours behind UTC |
Worked example: Summer in Phoenix
In July, it is 3:00 p.m. in Los Angeles. What time is it in Phoenix?
- In July, Los Angeles is on daylight saving time: 7 hours behind UTC.
- Phoenix stays on standard time: also 7 hours behind UTC.
- They match, so it is 3:00 p.m. in Phoenix too.
Answer: 3:00 p.m.
Common mistake
People think every state changes its clocks. Hawaii and most of Arizona do not. So in summer, Arizona has the same clock time as California, even though Arizona is in the Mountain zone. In winter, Arizona matches Colorado again.