The clock changes are where meetings go wrong
The gap between two zones is not fixed. The US changes clocks two to three weeks before Europe, so for a fortnight each spring and autumn the difference between New York and London is four hours rather than five. A recurring meeting set during one period silently moves during the other.
The southern hemisphere is offset by half a year, so the gap between London and Sydney varies between 8, 9, 10 and 11 hours over a year.
Abbreviations are ambiguous
"CST" is Central Standard Time in North America, China Standard Time, and Cuba Standard Time — three different offsets. "IST" covers India, Ireland and Israel. Naming a city, or using a UTC offset, removes the guesswork; naming an abbreviation introduces it.
The hours that do not exist
When clocks spring forward an hour is skipped entirely; when they fall back an hour happens twice. Scheduling something at 02:30 on a transition night gives you an appointment that either never occurs or occurs twice, which is a real source of duplicate reminders and missed calls.
Frequently asked questions
Why did my recurring meeting move by an hour?
The two countries changed clocks on different dates. The US switches two to three weeks before Europe, so the gap changes for a fortnight twice a year.
What does CST mean?
Three different things — Central Standard Time, China Standard Time and Cuba Standard Time. Use a city name or a UTC offset instead.
What happens to 02:30 on a clock-change night?
In spring it does not exist; in autumn it happens twice. Both cause real scheduling problems, so avoid that hour for anything important.
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