TimerPlanScheduling across time zones
The difference between two places is not fixed: the United States and Europe change their clocks on different dates, so the New York to London gap is five hours for most of the year and four for about three weeks each spring. Anything remembered as a constant is wrong during those weeks, and those are the weeks when meetings go missing.
The second failure is the day. Five in the afternoon in San Francisco is one in the morning in London tomorrow, and a time written without a date reads as though it were the same evening.
The third is abbreviations. CST is Central Standard Time, China Standard Time and Cuba Standard Time; IST covers India, Ireland and Israel. Named zones — America/Chicago, Asia/Kolkata — are the only unambiguous way to write one down.
The weeks to watch
| Change | Northern spring | Northern autumn |
|---|---|---|
| United States | Second Sunday in March | First Sunday in November |
| European Union and UK | Last Sunday in March | Last Sunday in October |
| Australia (southern) | First Sunday in April, back | First Sunday in October, forward |
| Countries with no change | — | Japan, India, China, most of Africa, Arizona |
The mismatch in March is the expensive one: for about three weeks the US is an hour closer to Europe than everybody’s memory says, and a recurring call set months earlier lands an hour out for one side.
Three habits that remove the problem
Convert once, in UTC. Put each person’s working day into UTC and compare there — the world meeting planner does the overlap arithmetic, and the time zone converter does the conversion.
Say the date. Not “five o’clock”, but “Tuesday 17:00 in London, which is Wednesday 02:00 in Sydney”. Half of all cross-zone errors are a correct time on the wrong day.
Name the zone properly. Europe/Berlin, not CET. Abbreviations are ambiguous, unofficial and change with the season — CET becomes CEST in summer, and half the people using it do not adjust.
Watching it rather than calculating it
For places you deal with daily, keep them on screen: the world clock shows local time, the date and whether it is the middle of the night there, which is the question actually being asked when someone reaches for a converter. For a single conversion, the time zone converter gives the time and the day, and works the offsets out for today rather than for whatever the rule was last year.
Frequently asked questions
Why do the US and Europe change clocks on different dates?
Because the two systems were legislated separately and never harmonised. The US moves on the second Sunday in March and the first Sunday in November; the EU moves on the last Sunday in March and the last in October. The result is roughly three weeks in spring and one in autumn when the usual gap is an hour different, and no software mistake is needed for a meeting to be missed.
What about the southern hemisphere?
It moves the opposite way, which doubles the effect. When European clocks go back in October, Australian clocks go forward, so the London to Sydney gap changes by two hours rather than one — from nine to eleven. Anybody scheduling regular calls between the hemispheres re-checks them twice a year or lets the calendar do it.
Should I write times in UTC?
For anything computed — logs, deadlines, systems talking to each other — yes, because UTC has no daylight saving and no ambiguity. For anything a person reads, write their local time and the date, because a colleague will not convert UTC in their head at seven in the morning and will get it wrong when they try.
Does my calendar handle this correctly?
Usually, provided every participant’s calendar has the right zone set and the invitation was created with a zone rather than a bare time. The common failure is a traveller whose laptop is on local time and whose phone is not, which produces two different reminders for the same meeting. Recurring meetings across a clock change are the other one: some tools keep the local time in each zone and some keep the interval.
What is a fair rotation for a global team?
Rotate which region takes the awkward hour rather than fixing one that suits the head office. If the overlap window is genuinely empty — which the world meeting planner will tell you — the choice is between an early start, a late finish and not meeting at all, and making that choice explicitly is better than defaulting to whoever complains least.
How this works
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