Why does my recurring meeting move by an hour twice a year?

Because a weekly slot fixes a local time, not 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. In the week between, the gap between them is an hour smaller than usual, so the meeting lands an hour later for one side.

Why is the shift two hours for Australia?

Because the southern hemisphere is on the opposite cycle — their clocks go forward when Europe's go back. The gap does not shift by an hour and return; it swings by a full two hours across the year.

Which zones never change their clocks?

Japan, India, China, most of sub-Saharan Africa, Arizona and Queensland among others. A meeting between two such zones is stable all year. A meeting between one of them and a zone that does change will always move, because only one end is moving.

What does the anchor zone mean?

The zone the meeting time is fixed in — whoever set it. That time never changes; everyone else's local time is what moves. Deciding which end is the anchor, and telling people, is most of the fix.

Does this account for countries changing their rules?

It uses your browser's time zone database, which is updated with the browser. Countries do occasionally change or abolish daylight saving with limited notice, so a date far in the future is a good prediction rather than a guarantee.

Is any of this sent anywhere?

No. Everything is worked out in your browser using its built-in time zone data. Nothing is uploaded and there is no account.

Recurring Meeting Clock-Change Checker

A weekly slot is a fixed local time, and a fixed local time is not a fixed moment. When one country changes its clocks and another has not yet, a standing meeting silently moves by an hour for everyone in the second country. This shows every date that happens on.

Every at set in for the next

Also attended from

Every occurrence

Highlighted rows are the weeks where the local time changes for somebody. Times shown in red fall outside 07:00–20:00.

Why a standing meeting moves

Book something for 09:00 every Tuesday in London and you have fixed a wall-clock time, not a moment. Everyone else's local time is derived from the gap between their zone and London — and that gap is not constant.

The gap changes because countries move their clocks on different dates. In autumn 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 between, the gap between London and New York is four hours instead of the usual five, so a 09:00 London meeting is 05:00 in New York rather than 04:00.

Nothing in a calendar invitation warns anybody. The entry says 09:00 and it still says 09:00. People in the other country simply turn up an hour out, or miss it entirely, and it happens twice a year to every recurring meeting that crosses those borders.

The southern hemisphere is worse

Australia, New Zealand, Chile and South Africa's neighbours are on the opposite cycle: their clocks go forward when Europe's go back. So the London–Sydney gap does not shift by an hour and come back — it swings a full two hours across the year, and there are short periods where it changes twice in a few weeks.

A meeting that works comfortably for both ends in June can be at half past six in the morning in November without anyone changing anything.

Zones that never change

Some places have no daylight saving at all: Japan, India, China, most of sub-Saharan Africa, Arizona, Queensland. Meetings between two such zones are stable all year, which makes them worth choosing as the anchor when you can.

It also means a meeting between a changing zone and a fixed one always moves — there is no arrangement of dates where both stay put, because only one of them is moving.

What to do about it

The fix is not clever, it is just deliberate. Decide in advance which end the meeting is anchored to and tell everyone, so when the gap changes people know whose clock is the one that governs. Then put the two or three dates in the year when the local time shifts into the invitation itself, and send a reminder in the week it happens.

Where a meeting is genuinely painful for one side all year, alternating between two slots is kinder than picking one that is unpleasant for the same people every week.