Daylight Saving Time in the United States

The short answer

In the United States, daylight saving time (DST) starts at 2:00 a.m. local time on the second Sunday of March and ends at 2:00 a.m. local time on the first Sunday of November. At the spring transition the clock jumps forward to 3:00 a.m.; at the fall transition it falls back to 1:00 a.m. and the 1-to-2 hour repeats. Hawaii and most of Arizona ignore the change. Every other U.S. state and the District of Columbia observe it.

Why it exists

The official rationale is energy use. By shifting the clock forward in spring, more daylight falls into the evening hours when people are awake and active, theoretically reducing the need for electric lighting. In practice, modern studies are mixed; the energy savings are small or negligible, and the original WWI-era and WWII-era arguments about coal use don't translate cleanly to a 21st-century grid that runs heating, cooling, and electronics around the clock.

What survives, regardless of the energy debate, is the cultural function: lighter evenings during the warm months. That single benefit is the reason DST has hung on through repeated attempts to abolish it.

The law that sets the dates

Federal authority over civil time in the United States comes from the Uniform Time Act of 1966, administered by the U.S. Department of Transportation. The Act sets uniform start and end dates for DST nationwide and lets states opt out (but not pick their own dates). The current second-Sunday-of-March / first-Sunday-of-November schedule was set by the Energy Policy Act of 2005, which extended DST by about four weeks compared to the previous schedule and took effect in 2007.

Because the dates are written into federal law, individual states cannot choose to start or end DST on different days. They can only choose between observing the federal schedule or staying on standard time year-round.

Who opts out

Hawaii has not observed DST since 1967. Its tropical latitude means daylight length doesn't change much across the year, so the case for shifting the clock was weak from the start.

Arizona opted out in 1968, with one exception: the Navajo Nation, which extends across parts of Arizona, New Mexico, and Utah, observes DST throughout. That creates a famous edge case in the spring and fall, when driving through certain highways in northern Arizona means crossing in and out of the same time depending on which jurisdiction you're in.

The U.S. territories of Puerto Rico, the U.S. Virgin Islands, Guam, the Northern Mariana Islands, and American Samoa also do not observe DST.

The exact transition mechanics

DST transitions happen at local 2:00 a.m., not simultaneously across the country. That detail matters more than people realize. When Eastern Time springs forward, the rest of the country is still on standard time for another hour or two. For roughly an hour each spring, the offset between Eastern and Central is one hour; for that same hour the offset between Eastern and Pacific is four hours instead of three. Software that hard-codes "ET is three hours ahead of PT" gets that hour wrong, every year.

Spring forward — what happens to 2:00 to 2:59

On the second Sunday of March, the moment the clock would tick to 2:00 a.m., it instead reads 3:00 a.m. The hour from 2:00 to 2:59 simply does not exist that day in observing zones. Times in that range are ambiguous in some software libraries and may be silently shifted, so anything you schedule "for 2:30 a.m." on that date is unlikely to behave the way you expect.

Fall back — the repeated hour

On the first Sunday of November, when the clock would tick to 2:00 a.m., it instead reads 1:00 a.m. The hour from 1:00 to 1:59 happens twice. To distinguish them, time databases mark the first occurrence with the daylight offset (e.g., EDT, UTC-4) and the second with the standard offset (e.g., EST, UTC-5). A 1:30 a.m. timestamp without the offset is genuinely ambiguous on that day.

Standard time vs daylight time

Each U.S. zone has two names — one for the half of the year on standard time and one for the half on daylight time. The distinction is the offset from UTC:

The full list of abbreviations and how to use them in writing is on the abbreviations reference.

Where current legislation stands

For several years Congress has debated the Sunshine Protection Act, a bill that would make daylight saving time permanent — that is, lock the clock on the "spring forward" side year-round, eliminating the November fall-back. The Senate has passed versions of the bill; the House has not enacted it, and as of this page's last review no permanent change has become federal law. Until that changes, the schedule above remains in effect.

Some states have passed their own laws to adopt permanent DST, but those laws are conditional on Congress amending the Uniform Time Act. States cannot unilaterally adopt permanent DST under current federal law; they can only adopt permanent standard time (as Arizona and Hawaii have).

Common DST mistakes

• Assuming "ET is always 3 hours ahead of PT." True for most of the year, but for a roughly two-week window in March and another in November, the offsets between zones don't all match the usual rule because zones transition on the same date but at local 2 a.m.
• Forgetting Arizona. If you have customers, family, or coworkers in Phoenix, half the year they're on Mountain Time and half the year they're effectively on Pacific Time without the clock changing on their end.
• Using "EST" all summer. "EST" specifically means Eastern Standard Time, which is November–March. From March to November, the correct abbreviation is "EDT" — or, if you want one term for both, "ET."
• Scheduling 2:30 a.m. on the second Sunday of March. That moment does not exist. Calendar systems handle it inconsistently. If you must schedule then, use 1:30 a.m. or 3:30 a.m.

If you're building software

The IANA Time Zone Database (often called tz or zoneinfo) is the authoritative source. Use zone identifiers like America/New_York, never offsets like UTC-5, when storing or scheduling future times. The offset is a property of a (zone, instant) pair; an offset alone loses the information needed to handle transitions correctly. Run library updates promptly when DST rules change — they have changed multiple times in U.S. history, and they may change again.

Quick checklist before the next transition

• Note the date in your calendar (second Sunday of March, first Sunday of November).
• Check that recurring meetings still hit the intended local time for every participant — particularly the week before and after a transition.
• Replace battery-operated clocks that don't auto-adjust, or label them so you know which way they drift.
• If you live near the Arizona / Navajo Nation boundary, reconfirm any cross-jurisdiction appointments.

For state-by-state detail on which zones apply where (and which counties don't follow the state's default), see the U.S. states time-zone reference. For a refresher on the live offsets, head back to the homepage — every clock there reflects the current DST status automatically.