Twice a year, a large part of the world moves its clocks by an hour — forward in spring, back in autumn. It is called daylight saving time, and while the goal is simple, the side effects reach into everything from sleep to software to that meeting invite that suddenly lands an hour off.
What daylight saving time is
Daylight saving time (DST) shifts the clock forward by one hour during the warmer months so that evenings have more daylight and mornings have less. When it ends, clocks move back to "standard time." Nothing about the sun changes — we simply relabel the hours to nudge our daily routine toward the light.
The two moments have friendly names: spring forward (clocks jump ahead, and you lose an hour of sleep) and fall back (clocks retreat, and you gain one). On the spring day, a clock skips straight from 1:59 to 3:00. In autumn, the hour from 1:00 to 2:00 happens twice.
A short history
The idea is often traced, half-jokingly, to Benjamin Franklin, who mused that waking earlier would save candles. The modern version arrived during the First World War, when several countries adopted it to conserve fuel. It spread, receded, and spread again through the 20th century, usually justified by energy savings — though modern studies find the actual savings small and sometimes negative.
Who changes their clocks, and who doesn't
This is where it gets messy. DST is far from universal:
- Much of North America and Europe observes it, but on different dates — so for a few weeks each year the usual offset between, say, New York and London is temporarily off by an hour.
- The Southern Hemisphere is flipped. When the north springs forward, places like parts of Australia and New Zealand are heading into autumn and doing the opposite.
- Most of Asia, Africa, and the tropics don't bother. Near the equator, day length barely changes across the year, so there is little to gain. India, China, Japan, and most of Africa keep one time all year.
- Some regions opted out even where neighbors observe it — Arizona in the United States famously does not change its clocks.
The trap this creates
Because regions switch on different dates — and hemispheres switch in opposite directions — the gap between two cities is not fixed. London and New York are usually 5 hours apart, but for a couple of weeks each spring and autumn they drift to 4 or 6. If you schedule by "we're always 5 hours apart," DST will eventually bite you.
Why it causes so many headaches
Beyond the lost hour of sleep — which research links to short-term spikes in accidents and health incidents — DST is a notorious source of scheduling errors. Recurring calendar events can land an hour off after a switch. Software that assumes a fixed offset silently breaks. International calls get missed. The fix in every case is the same: reason in named time zones (which know the DST rules), not fixed offsets.
Is DST going away?
There is a long-running push in several countries to abolish the twice-yearly change and stay on one time year-round. Proposals surface regularly; some have passed one legislative chamber or another. As of now, though, the change persists in most places that have long observed it, and any reform tends to move slowly. The safest assumption is that DST is still in force where it always has been — and to let your devices track the exact rules for you.
Never miscount an hour again
Our clocks and planner apply each city's real DST rules automatically — no mental math.