Why your recurring meeting moves an hour

A weekly slot fixes a local time, not a moment — and the gap between two countries is not constant.

The thing that actually happens

You book a call for 09:00 every Tuesday, London time. Someone in New York adds it to their calendar and it shows as 04:00. For months, nothing changes.

Then one Tuesday in autumn, half the people join at 04:00 and half at 05:00.

Nothing was edited. The invitation still says 09:00 London and it still says 04:00 New York in the calendar that created it. What changed is the gap between the two zones, and it changed because the two countries move their clocks on different dates.

Why the dates do not line up

The UK and the rest of Europe change on the last Sunday in October. The United States changes on the first Sunday in November. For the week or so between, the gap between London and New York is four hours instead of the usual five.

In spring it happens in the other order — the US moves in early March, Europe at the end of the month — so there is a longer window, several weeks, where the gap is wrong.

A calendar system will usually get the individual instant right. What it will not do is tell anybody. Nothing highlights the week where the local time changed, so people either notice on the day or miss the meeting.

The southern hemisphere is a bigger problem

Australia, New Zealand and Chile are on the opposite cycle: their clocks go forward when Europe's go back. The London–Sydney gap does not shift by an hour and return — it swings by a full two hours across the year.

And because the changes are weeks apart at each end, there are periods where the gap changes twice within a month. A meeting that is a civilised early evening in Sydney in June can be an unpleasant late night in November, with no one having touched it.

Zones that never change

Japan, India, China, most of sub-Saharan Africa, Arizona, Queensland, Singapore and the Gulf states have no daylight saving at all. Two such zones are stable relative to each other all year.

There is a consequence people find counter-intuitive: a meeting between a fixed zone and a changing one always moves. There is no arrangement of dates where both stay put, because only one of them is moving. India is a common case — the gap to London changes twice a year, every year, and it is always India's local time that shifts.

The half-hour and quarter-hour zones

Not every offset is a whole number of hours. India is five and a half hours ahead of UTC, Nepal five and three-quarters, parts of Australia nine and a half. Mental arithmetic that assumes whole hours will be wrong, and it will be wrong by an amount small enough that people talk past each other rather than immediately spotting the error.

What to do about it

None of the fixes are clever. They are just deliberate.

Decide which end anchors the meeting, and say so. If the time is fixed in London, then London is the truth and everyone else's local time is derived. When the gap changes, people know whose clock governs. Undocumented, every clock change turns into a discussion.

Put the shift dates in the invitation. There are only two or three a year. Listing them in the description — “this moves to 05:00 New York from 27 October to 3 November” — costs nothing and works, because people read the description when they are confused.

Send a reminder the week it happens. The one message that reliably prevents a half-empty call.

Alternate slots where one side always suffers. If the only overlap is unpleasant for the same people every week, two alternating times spread the cost instead of concentrating it.

Prefer a fixed-offset anchor for genuinely global meetings. Anchoring to UTC, or to a zone that never changes, removes the problem for everyone at once — at the cost of the meeting drifting by an hour for the people who do change their clocks, which at least is a single, predictable, well-understood shift.

Frequently asked questions

Why does my recurring meeting move by an hour?

Because a weekly slot fixes a local time rather than a moment, and countries change their clocks on different dates. Europe changes on the last Sunday in October, the United States on the first Sunday in November, so for the week between, the gap between them is an hour smaller than usual.

Why is the shift two hours for Australia and New Zealand?

Because the southern hemisphere is on the opposite cycle — their clocks go forward when Europe's go back. Rather than shifting an hour and returning, the gap swings a full two hours across the year.

Which countries do not change their clocks?

Japan, India, China, most of sub-Saharan Africa, Singapore, the Gulf states, Arizona and Queensland, among others. Two such zones are stable relative to each other all year.

If one zone never changes, does the meeting stay put?

No — the opposite. If one end changes and the other does not, the gap must change, so the meeting always moves for one side. Only two fixed zones give a stable arrangement.

Will my calendar not handle this automatically?

It will usually schedule the correct instant. What it will not do is tell anyone that their local time has changed for a few weeks. There is no highlight and no warning, which is why people arrive an hour out.

How far ahead can I trust these dates?

They come from your browser's time zone database, which updates with the browser. Countries do occasionally change or abolish daylight saving with limited notice, so distant dates are a good prediction rather than a guarantee.