Daylight Saving Time Explained: History, Impact, and Future

Twice a year, about 1.5 billion people change their clocks, and twice a year, a familiar argument breaks out about whether they should. Daylight saving time (DST) — the practice of shifting clocks forward by an hour in spring and back in autumn — was designed to save energy by aligning waking hours with daylight. A century of evidence suggests the savings are small and the costs are real. Here is how DST came to be, what it actually does, and why its future is uncertain.

Where daylight saving time came from

The idea of shifting clocks to capture more daylight is older than you might think. Benjamin Franklin half-jokingly suggested it in a 1784 satirical letter, proposing that Parisians wake earlier to save on candle wax. The serious proposal came from New Zealand entomologist George Hudson in 1895, who wanted more evening light to collect insects. But DST was not adopted nationally until World War I. Germany and its ally Austria-Hungary introduced it on April 30, 1916, to conserve coal for the war effort. Britain followed weeks later, and the United States adopted it in 1918. After the war, most countries dropped it, only to bring it back during World War II and again during the 1970s energy crisis.

The United States standardized the start and end dates in 1966 with the Uniform Time Act, which previously had been a patchwork of state and local rules. In 2007, the US shifted the dates to the second Sunday in March and the first Sunday in November, extending DST by about a month. The EU follows a different schedule — the last Sundays of March and October — which creates a few weeks each year when the US and Europe are out of sync.

What DST actually does to the clock

In DST, the clock is moved forward by one hour relative to standard time, so that sunrise and sunset both occur later by the clock. When DST ends, the clock moves back, giving an extra hour of sleep (and a 25-hour day). The transition in spring costs an hour, producing a 23-hour day. The names can be confusing:

• EST (Eastern Standard Time) = UTC−5, used in winter.
• EDT (Eastern Daylight Time) = UTC−4, used in summer.
• The IANA zone America/New_York handles both automatically, which is why you should always use the zone name, not the abbreviation.

For a full breakdown of how these offsets interact with UTC, see What Is UTC? and How Time Zones Work.

The health effects

The spring-forward transition is genuinely bad for human health. Studies have documented a spike in heart attacks, strokes, and traffic accidents in the days immediately after the March shift, attributed to the lost hour of sleep and the disruption of circadian rhythms. The effect is small in absolute terms but consistent across multiple studies. The autumn transition is gentler — an extra hour of sleep — but the twice-yearly reset itself is thought to contribute to chronic "social jet lag," a mismatch between the body's clock and the social clock.

Sleep researchers generally favor permanent standard time, which keeps the sun rising at a biologically reasonable hour. The American Academy of Sleep Medicine has explicitly endorsed permanent standard time, citing evidence that DST is misaligned with human circadian biology.

The economic effects

The original rationale — energy savings — is weaker than expected. A 2008 US Department of Energy study found DST reduced electricity use by about 0.03%, a real but tiny figure. Other studies have found that any savings on lighting are offset by increased heating in the cooler mornings and increased cooling in the warmer evenings. The economic effects that get more attention are the costs of the transition itself: lost productivity, increased accident rates, and the engineering burden of updating software and systems twice a year.

The engineering headache

For software, DST is a recurring source of bugs. A few classic examples:

• The 23-hour and 25-hour days. A recurring meeting at 2:30 AM may not exist on the spring-forward day (the clock jumps from 2:00 to 3:00) and may occur twice on the fall-back day. Code that assumes every day has 24 hours breaks.
• Out-of-sync regions. Arizona does not observe DST, but the Navajo Nation inside Arizona does, and the Hopi Reservation inside the Navajo Nation does not. Scheduling software that does not use the IANA database can place a meeting in the wrong zone.
• US/EU drift. For a few weeks each spring and autumn, the US and EU are on different DST schedules, shifting the gap between, say, New York and London by an hour. Recurring transatlantic meetings break during those windows.

Our Timezone Converter and Meeting Planner read the IANA database, so they handle these transitions correctly — but only if the underlying system's tzdata is up to date.

Will DST end?

There is real momentum toward abolition. The European Parliament voted in 2019 to end DST in the EU, though implementation has stalled as member states disagree on whether to land on permanent summer time or permanent winter time. In the US, the Senate passed the Sunshine Protection Act unanimously in 2022, which would make DST permanent, but the bill has not advanced in the House. A growing number of US states have passed laws to stay on permanent DST, but they cannot take effect without federal authorization. The direction of travel seems clear — fewer clock changes — but the destination (permanent summer time or permanent winter time) is still being argued.

How to cope in the meantime

1. Adjust your sleep gradually. Shift your bedtime by 15 minutes a day for the four days before the transition to soften the jet lag.
2. Audit recurring meetings around transition dates. A weekly call that works in February may suddenly fall at an inconvenient hour in March.
3. Use IANA zone names in every system you build. Never store "EST" or "PST" as a fixed offset; store America/New_York or America/Los_Angeles and let the database resolve the offset for the date.
4. Keep tzdata updated. A server that has not updated its time-zone database will eventually produce wrong times as rules change.

The bottom line

Daylight saving time is a century-old energy policy that has outlived its original justification. It disrupts sleep, complicates software, and produces no meaningful energy savings by most modern estimates. The political will to abolish it is growing, but until then, the twice-yearly ritual continues — and the best thing you can do is build systems that handle it gracefully.