How Daylight Saving Time Breaks Your Schedule Twice a Year
Which regions shift their clocks and when, why international time gaps wobble in March and October, and how to schedule around it.
What actually happens
Daylight saving time moves a region's clocks forward one hour in spring and back one hour in autumn, trading dark mornings for lighter evenings during the summer half of the year. The change happens on a scheduled night, usually at 1 or 2 AM local time, and the region's UTC offset shifts by one hour until the reverse change.
Crucially, this is a regional policy, not a global event. When New York "springs forward", Seoul does nothing — so the gap between them changes by an hour overnight, even though nothing happened in Korea at all.
Who shifts and who does not
Most of North America and all of the EU observe daylight saving. Most of Asia does not: Japan, South Korea, China, India, and Singapore hold the same offset all year. Most of Africa and the Middle East skip it too. In the southern hemisphere, parts of Australia, New Zealand, and Chile observe it — but their summer is the northern winter, so they shift in the opposite months.
Even within one country the rule can vary. In Australia, Sydney and Melbourne shift while Brisbane and Perth do not; in the US, Arizona and Hawaii opt out. The safe assumption is that daylight saving status is a property of a city, not a country.
The dangerous weeks
The US moves its clocks on the second Sunday of March and the first Sunday of November. Europe moves on the last Sunday of March and the last Sunday of October. Those mismatched dates create two or three "limbo weeks" each year when the London–New York gap is 4 hours instead of the usual 5 — and every standing meeting between the two lands an hour off for exactly that stretch.
Pairs involving the southern hemisphere wobble even more. London–Sydney cycles through 9, 10, and 11 hours across a year because the two cities shift in opposite seasons. If a recurring call spans hemispheres, put both transition periods in your calendar as review reminders.
Scheduling defensively
Three habits absorb almost all daylight saving damage. First, create calendar events in the participant's actual timezone rather than typing a converted time — calendar software then adjusts automatically when either side shifts. Second, quote times with a city attached ("14:00 Berlin time"), never with a bare offset like "UTC+1", because the offset itself is what changes. Third, re-verify any standing international meeting in mid-March and late October before it fails on its own.
And when a colleague abroad suddenly seems an hour late or early in those weeks, check the calendar before assuming a mistake — the clocks moved, not the person.
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