Why a converted time shifts by an hour twice a year

People who work across zones tend to carry one number in their head for each pair of cities they deal with. London to New York is five. Copenhagen to San Francisco is nine. The number is right most of the year, which is exactly what makes it dangerous: it is wrong for a few weeks each spring and autumn, and those weeks arrive without anyone announcing them.

The gap is a subtraction of two moving numbers

A city does not have a difference from another city. It has an offset from UTC, the world's reference clock, and the difference you care about is one offset minus the other. When either place changes its offset, the difference changes with it. A place that observes daylight saving spends part of the year one hour further from UTC than the rest, so any pair where one side changes and the other does not will shift by an hour twice a year.

That much is familiar. The part that surprises people is what happens when both sides change, but not on the same date.

Why the anomaly lasts weeks, not days

Europe and North America both move their clocks in spring and autumn, and both move them by an hour, but the changeover weekends do not coincide. North America springs forward earlier in March than Europe does, and falls back later in autumn. In the gaps, one continent has already moved and the other has not, so the usual difference is temporarily an hour off.

The southern hemisphere adds a second pattern. Australia and New Zealand are entering summer when Europe is entering winter, so their clocks move in the opposite direction at roughly the opposite time of year. The difference between London and Sydney is not a number at all in any useful sense: it moves through a two-hour range over the course of a year.

What breaks in practice

• Standing meetings. A weekly call fixed by local time on both sides will quietly move relative to one of them. Someone loses an hour of their morning, and it usually takes a fortnight for anyone to say so.
• Cached conversions. A difference written into a document, a wiki page or a signature block is a snapshot. It ages badly and nothing flags it.
• Flight and delivery windows. Anything expressed as "arrives six hours later" rather than as a local arrival time will drift in the changeover weeks.

How to stay ahead of it

Anchor on the instant rather than the gap. If you agree a moment in UTC, no clock change can move it: only the local readings shift, and each side reads it correctly for themselves. Calendar software does this natively when an invite carries a proper time zone, which is why an invite is a better artefact than a message saying "same time as usual".

When you do need the gap in words, write it in a season-stable form. "Five hours in winter and four in the changeover week" is more useful than "five hours", and "UTC-5 in winter, UTC-4 in summer" is more useful still, because it says what the place does rather than what it happens to be doing today.

And when you simply want the right answer for a specific date, look it up rather than remembering it. The time zone converter evaluates both offsets for the date you give it, using the same IANA database that operating systems use, so the awkward fortnights are handled without you having to know they exist.