What Is Daylight Saving Time (DST)?

How Daylight Saving Time Works

In the US, clocks jump from 2:00 AM to 3:00 AM in spring, so that hour “disappears.” In autumn they go from 2:00 AM back to 1:00 AM, so that hour happens twice. In the UK and EU the change happens at 1:00 AM UTC, which is 1:00 AM to 2:00 AM in London in spring.

Sunsets land an hour later on the clock all summer, then return to earlier times in winter. The effect on daily life is mostly about evening light.

Where It Came From

Benjamin Franklin gets the credit in popular tellings, but his 1784 letter was satire, not a proposal. The modern idea came from George Hudson in New Zealand in 1895 and William Willett in Britain in 1907. Germany and Austria-Hungary were the first to adopt it nationally, on April 30, 1916, to conserve coal during the First World War. The US followed in 1918, and the Uniform Time Act of 1966 standardized it.

Saving energy is the usual justification today, but the evidence is mixed. A 2017 analysis of 44 studies found small electricity savings of about 0.3 percent on DST days.

How DST Affects UTC Offsets

UTC itself never changes. What changes is the offset that a local timezone applies to UTC.

The Missing Hour and the Repeated Hour

On the spring-forward day, local times from 2:00 to 2:59 AM don't exist in the US. On the fall-back day, 1:00 to 1:59 AM happens twice. A meeting at 2:30 AM, or a recurring job scheduled in that window, can be skipped, shifted, or run twice, depending on the software.

That's one reason developers store timestamps in UTC and convert to local time only for display. UTC has no missing or repeated hours.

When Countries Don't Switch on the Same Day

Because the US starts earlier and Europe ends earlier, the two are out of step for a few weeks each year. In spring the US moves first, by two or three weeks depending on the year. In autumn Europe moves back exactly one week before the US does.

During those windows the New York to London gap is 4 hours instead of the usual 5. Australia and Chile are in the Southern Hemisphere, so their DST runs roughly October to April. In Australia, only four states and one territory observe it.

Which Countries Don't Use DST?

Most of the world doesn't use it. About 70 countries do, roughly a third of all countries, mostly in Europe and North America. India, China, Japan, South Korea, Singapore, the UAE, most of Africa, and most of Southeast Asia stay on a fixed offset all year.

In the US, Hawaii and most of Arizona opt out. Some countries have dropped it: Russia moved to permanent standard time in 2014, and Mexico ended seasonal changes for most of the country in 2022. In the EU, the Parliament voted in 2019 to end clock changes, but member states still observe DST. See the DST 2026 dates page for this year's exact switch dates across all major regions.

Frequently Asked Questions

Related Tools & Terms