Daylight Saving Time, Explained
Twice a year a large fraction of the world moves its clocks, and twice a year a predictable wave of missed meetings, misfiring alarms and scheduling bugs follows. The practice is more recent, less universal, and less clearly beneficial than most people assume.
What it actually does
Daylight saving time shifts clocks forward by an hour in spring and back in autumn. No daylight is created. The clock is relabelled so that ordinary waking hours line up with a later sunset, trading a darker morning for a lighter evening.
Because it is a relabelling, the changeover days are not 24 hours long. The spring day has 23 hours, the autumn day has 25, and one local hour is either skipped entirely or repeated.
Who invented it (not Benjamin Franklin)
Franklin is routinely credited and does not deserve it. His 1784 letter to a Paris journal — proposing that Parisians be woken at dawn by church bells and cannon fire to save on candles — was satire, and it proposed changing habits, not clocks.
The real proposals came later. George Hudson, a New Zealand entomologist who wanted more daylight after work to collect insects, put a two-hour shift to a scientific society in 1895. In Britain, the builder William Willett campaigned from 1907 for clocks to move in a series of small steps, and lobbied until his death in 1915 without success.
Adoption came from war, not persuasion. Germany and Austria-Hungary introduced it on 30 April 1916 to conserve coal. Britain followed within weeks, and the United States in 1918. It was then repealed, reintroduced, and repealed again in various countries for decades — the patchwork of local rules that resulted is why standardised timezone data became necessary in the first place.
Who observes it today
Roughly a third of the world's countries, concentrated in Europe, North America, and parts of South America and Oceania. Most of Africa and Asia do not observe it at all, and countries near the equator have little reason to, since day length barely varies.
Current rules differ by region. In the United States, clocks move forward on the second Sunday in March and back on the first Sunday in November, with each transition at 2am local time — so the change rolls across the country zone by zone. The European Union changes on the last Sunday in March and the last Sunday in October, at the same UTC instant everywhere, so the whole bloc shifts simultaneously.
Two more wrinkles catch people out. Southern hemisphere countries that observe it run on the opposite schedule, so Australia and the United Kingdom spend part of the year an unexpected number of hours apart. And observance can vary within a country: in the United States, Hawaii and most of Arizona stay on standard time year-round, while the Navajo Nation within Arizona does change.
Does it work?
The original case was energy. Modern evidence for that is weak: studies since the 2000s have generally found effects in the range of a fraction of a percent of electricity use, sometimes negative, because reduced evening lighting is offset by heating and air-conditioning demand. Lighting is also a far smaller share of household energy than it was in 1916.
The clearer effects are on people. Research has associated the spring transition — the one that removes an hour of sleep — with short-term increases in heart attacks, strokes, and road traffic collisions in the days immediately after the change. These are population-level statistical associations from observational studies rather than demonstrations of cause in any individual, and effect sizes vary between studies, but the direction is consistent enough that medical bodies have cited it in arguing for abolition.
Set against that, the arguments in favour are mostly about lighter evenings: more usable daylight after work, which is popular, and which is a preference rather than a finding.
Why it breaks software
The clock change produces two local times that behave badly, and a great deal of code assumes neither exists.
- A gap. In spring, local clocks jump from 01:59 to 03:00. A time like 02:30 simply does not occur. Anything scheduled for it must be nudged, skipped, or rejected — and the library you are using has an opinion about which, whether or not you share it.
- A fold. In autumn, 01:30 happens twice. A timestamp without an offset is genuinely ambiguous, and "the first one" versus "the second one" is a real distinction for logs, billing, and anything ordered by time.
Beyond those, recurring events drift. A weekly call agreed as 9am in London and 10am in Berlin holds all year, because both zones change together. The same call agreed between London and New York breaks for two weeks twice a year, because the US and EU change on different dates. A cached UTC offset also goes stale the moment the rules change, which is why offsets should be derived from a zone identifier at the time of use rather than stored.
The defence is the same as for time zones generally: store instants in UTC, store future appointments as a zone plus a local time, and let a maintained database resolve the offset when you need it.
Will it end?
Possibly, slowly. The European Parliament voted in 2019 to abolish seasonal clock changes, leaving each member state to choose a permanent standard or summer time; implementation stalled, partly because a patchwork of neighbouring countries on different permanent settings would be worse than the status quo. In the United States, the Sunshine Protection Act passed the Senate in 2022 but was not enacted.
The deadlock is not really about whether to stop changing clocks — it is about which time to keep. Permanent summer time means dark winter mornings; permanent standard time means very early summer sunrises. Sleep researchers generally favour permanent standard time as the closer match to solar time; retail and leisure interests generally favour the lighter evenings. Until that is settled, the clocks keep moving.
For working across zones while this is unresolved, the World Time page reads current local time directly from your system's timezone data, so it follows whatever rules are actually in force.