夏時間が会議に与える影響
この記事の全文は以下の英語版でご覧いただけます。
Daylight Saving Time (DST) is the quiet disruptor of global meetings. Twice a year, clocks shift — and for a week or two, teams on opposite sides of the change are suddenly an hour off from what their calendars said. Here's how DST actually affects your meetings and how to avoid the chaos.
Why DST Breaks Schedules
Most of North America and Europe observe DST, but they switch on different dates: the US changes in early March and early November, while the EU changes in late March and late October. For the gap weeks, the usual offset between, say, London and New York is off by an hour.
The "Limbo" Windows
| Period | London–New York gap | Risk |
|---|---|---|
| Early March (US springs forward, EU hasn't) | 4 hours | EU meetings run 1h late for US |
| Late March (EU springs forward) | 5 hours (normal) | Resolved |
| Late October (EU falls back, US hasn't) | 4 hours | US meetings run 1h early for EU |
| Early November (US falls back) | 5 hours (normal) | Resolved |
How to Protect Your Meetings
- Always schedule in UTC or a named zone. "9 AM ET" is unambiguous; "9 AM local" is not during transitions.
- Send calendar invites with timezone embedded. Google/Outlook handle the conversion — but only if the timezone is set on the event.
- Flag the transition weeks. Add a note to recurring meetings in March and October/November.
- Verify with a planner. Our Meeting Planner uses real DST rules, so it won't lie during the limbo weeks.
Frequently Asked Questions
Why do US and EU DST dates differ?
Each region sets its own law. The US (Energy Policy Act 2005) and the EU Directive 2000/84/EC chose different start/end Sundays.
How long is the "wrong offset" period?
About 1–2 weeks in spring (March) and autumn (October–November), until the second region also switches.
Do all US states observe DST?
Most do. Arizona (except Navajo Nation) and Hawaii do not. So a US internal meeting can still hit a 1-hour mismatch.
How do I avoid DST meeting errors?
Schedule in a fixed zone (UTC or ET), set the timezone on the invite, and double-check during transition weeks.
Which countries don't use DST at all?
Most of Asia, Africa, and parts of Oceania skip DST. See our list of places without DST.
Use our world clock and meeting planner to navigate DST correctly.
米国とEUの夏時間が異なるのはなぜですか？
各地域は独自の法律を定めています。米国（ 2005年エネルギー政策法）とEU指令2000/84/ECは、異なる日曜日の開始/終了を選択しました。
「間違ったオフセット」期間はどのくらいですか？
春（ 3月）と秋（ 10月〜11月）の約1〜2週間で、2番目の地域も切り替わります。
米国のすべての州が夏時間を遵守していますか？
ほとんどがそうです。アリゾナ州（ナバホ族を除く）とハワイ州はそうではありません。したがって、米国の社内会議でも1時間のミスマッチが発生する可能性があります。
夏時間のミーティングエラーを回避するにはどうすればよいですか？
固定されたゾーン（ UTCまたはET ）でスケジュールし、招待状にタイムゾーンを設定し、移行期間中に再確認します。
夏時間をまったく使用していない国はどこですか？
アジア、アフリカ、オセアニアの一部のほとんどは夏時間をスキップしています。夏時間のない場所のリストをご覧ください。