🕰When do the clocks change?

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The four rulebooks

United States and Canada. 15 U.S.C. 260a, as amended by the Energy Policy Act of 2005: daylight saving time begins on the second Sunday in March and ends on the first Sunday in November, at 2:00 a.m. local time. Because the trigger is local, the change rolls across the country zone by zone rather than happening at once. Canada’s provinces are not bound by the American statute but nearly all of them match it, so the two countries change on the same weekends — with Saskatchewan, which stays on standard time, as the main exception.

European Union. Directive 2000/84/EC: the last Sunday in March to the last Sunday in October, at 01:00 UTC. The trigger is a single instant, so the whole union switches together at different local hours.

United Kingdom and Ireland. The same two Sundays, under the Summer Time Order 2002. British Summer Time begins at 1 a.m. and ends at 2 a.m. local, which is 01:00 UTC on both occasions — the same instant as the continent.

Australia and New Zealand. Southern-hemisphere summer, so the season is inverted: New South Wales, Victoria, South Australia, Tasmania and the ACT run from the first Sunday in October to the first Sunday in April, and New Zealand from the last Sunday in September to the first Sunday in April.

Ten years of American clock changes

The date column is the spring change, the fourth column the autumn one. Both are computed from the statute at build time rather than copied from a list, so they extend as far as anyone needs. For the European dates, switch the calculator above to European Union — the result panel also shows the other side of the Atlantic and the gap between the two, which is the number that actually matters if you are scheduling across it.

Where the clocks never move at all

Roughly two-thirds of the world’s countries do not change their clocks, and the exceptions inside countries that do are the ones that catch people out.

In the United States, Hawaii and most of Arizona stay on standard time all year — but the Navajo Nation within Arizona does observe daylight saving, and the Hopi Reservation inside it does not. Puerto Rico, Guam, the US Virgin Islands and American Samoa do not change either.

In Australia, Queensland, Western Australia and the Northern Territory keep standard time, so the country runs on five different offsets for part of the year and three for the rest. Most of Asia, Africa and South America abandoned the practice or never adopted it.

The hour that does not exist

On the spring weekend a local hour is deleted — 2:00 a.m. jumps to 3:00 a.m. in the American rule — and on the autumn weekend an hour is lived through twice. Both create real problems that have nothing to do with sleep:

• A meeting set for the missing hour has no valid time. Calendar software resolves it, and different programs resolve it differently.
• Timestamps repeat. In the autumn, two different moments share the same local clock reading, so any log or record that stores local time without an offset becomes ambiguous for one hour a year.
• A night shift is 23 or 25 hours long. Payroll built on “clock out minus clock in” is wrong twice a year unless it works in UTC.
• Recurring cross-border calls slip. Nothing on either calendar changed; the offset between the two did, for a fortnight.

If you need whole-day arithmetic across one of these weekends, the days between dates tool counts calendar days between midnights, so a clock change inside the range cannot add or remove a day. For the working-day version, use the business days calculator.

The clock change is one of four kinds of moving date on this site. The others are the computus behind when Easter falls, the astronomy behind the season start dates, and the weekday rules behind holiday observed dates — all of them collected in the holiday dates group.