The core problem: regions don't switch together
Daylight saving time itself is simple — clocks jump forward an hour in spring, back an hour in autumn. The chaos comes from the fact that different regions switch on different dates, and some don't switch at all:
| Region | Springs forward | Falls back |
|---|---|---|
| US & Canada (most) | 2nd Sunday of March | 1st Sunday of November |
| Europe (EU & UK) | Last Sunday of March | Last Sunday of October |
| Australia (southeast) | 1st Sunday of October | 1st Sunday of April |
| New Zealand | Last Sunday of September | 1st Sunday of April |
| India, China, Japan, Singapore | No DST — offsets never change | |
| Arizona (most), Hawaii, Queensland | No DST — even though their countries observe it | |
Rules also change: countries regularly adopt, abolish, or move DST by legislation. Never trust a memorized offset across years.
The danger weeks
Because the US and Europe switch on different Sundays, there are two windows every year where the usual offset is wrong:
- Mid-to-late March (~2–3 weeks): the US has sprung forward, Europe hasn't. New York–London is temporarily 4 hours instead of 5. Your 15:00 London standing call with NY at 10:00 becomes NY at 11:00.
- Late October–early November (~1 week): Europe has fallen back, the US hasn't. The gap is temporarily 4 hours again, in the other direction.
Southern-hemisphere pairs are worse: Sydney and London move in opposite directions, so their offset changes by two hours across the changeover season (9, 10, or 11 hours depending on the date).
Who actually moves — you or them?
A recurring meeting is pinned to one timezone (usually the organizer's). When that zone changes its clocks and yours doesn't — or changes on a different date — the meeting moves in your day. This is why "the standup moved an hour, nobody told me" happens: nobody did anything; the calendar did exactly what it was told.
How to schedule through DST safely
- Pin recurring meetings to one named city ("10:00 New York time") and let everyone else's calendar convert. One source of truth, moved for everyone consistently.
- Mark the danger weeks. Each March and late October, spend one minute confirming your cross-region meetings — especially US–Europe and anything involving Australia.
- Never say "EST" or "CET" in summer. Eastern time is EDT half the year; central Europe is CEST. If you must use abbreviations, UTC offsets are safer — but city names beat both.
- Check live, not from memory. A menu bar clock that tracks the IANA timezone database — like GlobalTimeZones — always shows the current offset for every city, and its time slider lets you preview what a proposed slot means locally on the actual date, DST included.
Why DST exists at all (briefly)
Introduced widely during World War I to save fuel by matching waking hours to daylight, DST persists mostly by inertia; energy-saving evidence today is weak and both the EU and US have repeatedly debated abolishing it without following through. Until someone actually does, assume two clock changes a year — on dates that don't line up.