How to schedule a meeting across time zones

The mistake is almost never the arithmetic. It is applying today’s offset to a date after the clocks change.

Converting a time between zones looks like subtraction. It is, until daylight saving gets involved — and then the answer depends on which date you are asking about.

The offset is not a property of the zone

London is UTC+0 in January and UTC+1 in July. New York is UTC−5 and then UTC−4. Ask "what is 3pm London in New York?" and there is no single answer: it depends on the date, and on whether the two zones have changed their clocks yet.

Most converters take today's offset and apply it to whatever date you type. Book a meeting for after a clock change and it lands an hour out — which is exactly when it matters, because everybody has already put it in their calendar.

The two weeks when nothing lines up

Europe and North America do not change their clocks on the same day. Europe changes on the last Sunday of March and October; the United States on the second Sunday of March and the first Sunday of November.

That leaves roughly two weeks in spring and one in autumn when the usual gap between London and New York is not five hours but four or six. Recurring meetings drift by an hour, twice a year, and the calendar invite looks unchanged.

The southern hemisphere goes the other way

Sydney's clocks go forward when Europe's go back. The gap between London and Sydney varies between 8, 9, 10 and 11 hours across the year. A standing call between the two is the hardest recurring meeting there is.

Plenty of zones do not change at all

Japan, India, China and most of Africa keep one offset all year. When you are the one changing and they are not, the meeting moves for you and not for them — and it is worth saying so, because the person who did not move often assumes you rescheduled.

What actually works

• Agree in UTC for anything technical. A maintenance window at 02:00 UTC is unambiguous everywhere and does not shift.
• Send a calendar invite rather than a time in an email. Calendar software stores the instant and renders each person's local time correctly.
• Name the zone whenever you do write a time. "3pm" is not information.
• Check recurring meetings after each change. They are the ones that quietly move.
• Avoid the ambiguous hour for anything scheduled: when clocks go back, 01:30 happens twice in that zone.

Frequently asked questions

Why does my meeting time shift twice a year?

Because the two zones change their clocks on different dates. Europe changes on the last Sunday of March and October; the US on the second Sunday of March and first of November, leaving weeks where the usual gap is an hour different.

Should I schedule in UTC?

For anything technical, yes — it never changes, so a window at 02:00 UTC is unambiguous. For meetings with people, a calendar invite is better, because it renders correctly for everyone.

Why do converters give the wrong answer for a future date?

Most apply today's offset rather than the offset on the date you asked about. If a clock change falls between the two, the answer is an hour out.

What is wrong with writing CST or IST?

They are ambiguous. CST is Central Standard Time, China Standard Time and Cuba Standard Time; IST is India, Ireland and Israel. Use the full zone name or UTC.

Which zones do not observe daylight saving?

Japan, India, China, most of Africa, and much of Asia. If you change and they do not, the meeting moves for you alone.