Daylight Saving Time Explained: Why We Change the Clocks

Twice a year, hundreds of millions of people move their clocks by an hour — losing sleep in spring and gaining it in autumn. The practice is loved, hated, and increasingly questioned. Here's how daylight saving time really works, where it applies, and why a growing number of places are abandoning it.

What daylight saving time is

Daylight saving time (DST), called "summer time" in much of the world, is the practice of setting clocks forward by one hour in spring and back by one hour in autumn. The goal is to shift an hour of daylight from the early morning — when many people are still asleep — to the evening, when they can use it. During DST, a region's offset from UTC changes by one hour. New York, for example, runs at UTC−5 in winter and UTC−4 in summer.

"Spring forward, fall back"

The classic memory aid sums up the whole system:

• Spring forward — in spring, clocks jump ahead one hour. The change usually happens overnight, so 2:00 AM instantly becomes 3:00 AM. That night is one hour shorter, which is why the spring switch is the one people feel.
• Fall back — in autumn, clocks move back one hour, so 2:00 AM becomes 1:00 AM. You effectively get an extra hour, and that day is 25 hours long.

When does it start and end?

The exact dates differ by country, which is a frequent source of confusion:

• United States & Canada — DST runs from the second Sunday in March to the first Sunday in November.
• European Union & UK — "summer time" runs from the last Sunday in March to the last Sunday in October.
• Southern Hemisphere — because the seasons are reversed, countries like Australia, New Zealand, Chile, and parts of Brazil's past schedule shift in the opposite months, roughly October to April.

This mismatch means that for a few weeks each year, the time difference between, say, New York and London is briefly one hour off from its usual value — a classic trap when scheduling international calls. A time zone converter that accounts for DST automatically saves you from that mistake.

A short history

The idea is often credited to Benjamin Franklin, who joked in 1784 that Parisians could save candles by waking earlier. The first serious proposals came over a century later, and DST was first adopted widely during World War I as a fuel-saving measure. It has been switched on and off many times since, usually tied to energy concerns. The modern, standardized schedules most countries follow today were largely set in the 1970s and 2000s.

Does it actually save energy?

This is where the debate heats up. The original justification — using less artificial light in the evening — made sense in an era when lighting dominated household electricity use. Today, lighting is a small share of energy consumption, and modern studies find the savings are tiny or nonexistent. Some research even suggests increased use of heating and air conditioning cancels out any lighting savings entirely.

Meanwhile, the costs have become clearer. The spring clock change is linked to a short-term rise in heart attacks, traffic accidents, and workplace injuries, as people lose an hour of sleep and their body clocks fall out of sync. These health findings are a major reason the practice is now under review in many places.

Who doesn't use DST?

Most of the world's population actually does not change their clocks. Countries near the equator have little seasonal variation in daylight, so DST offers no benefit. Among the places that skip it:

• Most of Asia, including Japan, China, and India
• Most of Africa
• Most of South America near the equator
• In the United States, Hawaii and most of Arizona

The move to abolish it

Pressure to end clock-changing is growing. The European Union voted in 2019 to scrap mandatory seasonal changes, though implementation has stalled. In the United States, repeated proposals aim to make one time permanent year-round. The sticking point is which one to keep: permanent summer time means darker winter mornings, while permanent standard time means earlier sunsets in summer — so there's no option everyone agrees on.

How to stay on the right time

Your phone and computer handle DST automatically as long as their time zone is set correctly and "set time automatically" is enabled. When you're coordinating across borders, don't do the math in your head during the transition weeks — let a converter that knows each region's rules do it. You can check the precise current time anywhere with the world clock, and convert between zones with the time zone converter.