What is daylight saving time?

Daylight Saving Time (DST) is the practice of moving clocks forward by one hour during warmer months so that darkness falls at a later clock hour in the evening.

Why do clocks change for daylight saving time?

Clocks are adjusted forward in spring to transfer an hour of early morning daylight into the evening, with the historical goal of saving energy and encouraging outdoor activity after the workday.

When does daylight saving time start and end?

Transition dates vary by jurisdiction. In the United States and Canada, DST starts on the second Sunday in March and ends on the first Sunday in November. In the European Union and the United Kingdom, Summer Time starts on the last Sunday in March and ends on the last Sunday in October.

Which countries observe daylight saving time?

Approximately 70 countries observe DST, predominantly in North America, Europe, parts of the Middle East, and southern parts of Australia and New Zealand. Most nations near the equator and across Asia do not observe DST.

Does daylight saving time change the UTC offset?

Yes. When a region activates daylight saving time, its civil UTC offset advances by one hour (for example, US Eastern Standard Time at UTC-5 advances to Eastern Daylight Time at UTC-4).

Why does daylight saving time affect the time difference between cities?

Because regions enter and leave daylight saving on different calendar dates, the time gap between two international cities can expand or shrink by an hour during transition weeks in spring and autumn.

Does every country use daylight saving time?

No. The majority of the world population does not use daylight saving time. Countries near the equator experience consistent day length throughout the year, making seasonal clock changes unnecessary.

夏時間（サマータイム / DST）徹底解説：時計の変更時期と世界のルール

Daylight Saving Time (DST) is the practice of advancing clocks typically by one hour during warmer spring months and resetting them back to standard winter time in autumn to extend evening daylight.

「春は1時間進め、秋は1時間戻す（Spring Forward, Fall Back）」

During the transition into DST, clocks advance by 1 hour (e.g. from 01:59:59 AM to 03:00:00 AM), effectively "losing" an hour of sleep in exchange for more evening sunlight. In autumn, clocks fall back 1 hour, repeating the hour from 01:00 to 02:00.

夏時間を採用している国・廃止した国

North America: Most of the United States (excluding Arizona and Hawaii) and Canada observe DST from the second Sunday in March to the first Sunday in November (check current New York time).
Europe: European Union countries observe Summer Time from the last Sunday in March to the last Sunday in October (see London time and Paris time).
Southern Hemisphere: Australia (NSW, Victoria, SA, Tasmania) and New Zealand observe DST during their summer months (October to April), shifting clocks in the opposite direction from Europe and North America (compare Sydney time and Auckland time).
Non-Observing Nations: Most countries near the equator (such as India, Singapore, Indonesia, and Kenya), as well as China and Japan, do not observe DST because daylight hours vary little throughout the year (see Singapore time and Tokyo time).

大西洋間の夏時間移行ギャップ（国際会議設定時の重大な落とし穴）

One of the most frequent sources of missed business meetings and financial market confusion is the transatlantic shift discrepancy. The United States transitions into DST on the second Sunday in March and reverts on the first Sunday in November. In contrast, the European Union and the United Kingdom transition on the last Sunday in March and revert on the last Sunday in October.

As a result, for 2 to 3 weeks in March and 1 week in late autumn, the standard 5-hour time difference between New York (EDT/EST) and London (BST/GMT) shrinks to 4 hours. Global team schedulers must verify meeting times during these windows using our Global Meeting Planner or check exact dates on the Time Zone Converter.

南半球における季節の逆転と夏時間

Because the seasons are reversed in the Southern Hemisphere, countries like Australia and New Zealand observe daylight saving during their summer (October through April). When cities in North America or Europe "spring forward" by one hour, cities like Sydney or Melbourne are preparing to "fall back" by one hour. This dual shift creates dramatic changes in time differences—for example, the time difference between New York and Sydney varies between 14 hours and 16 hours depending on the month of the year.

Check Current DST Status & Convert Clocks

Our real-time tools automatically detect whether Daylight Saving Time is currently active for every municipality, updating offsets and solar timelines dynamically.

Open World Clock → Convert Time Zones → What is UTC? →

⏰ Daylight Saving Time FAQ

よくある質問 (FAQ)

Clear answers on how seasonal clock changes, transition schedules, and regional offsets work.

夏時間（サマータイム/DST）とは何ですか？

日照時間の長い春から秋にかけて時計を1時間進めることで、夕方の暗くなる時刻を遅らせ、自然光を有効利用する制度です。

なぜ夏時間のために時計の針を変更するのですか？

早朝の使われない自然光を夕方にシフトさせ、照明エネルギーの節約や仕事帰りの屋外活動を促進することを歴史的な目的として導入されました。

夏時間はいつ始まり、いつ終わりますか？

地域によって異なります。アメリカとカナダでは3月第2日曜日に始まり11月第1日曜日に終了します。欧州連合（EU）やイギリスでは3月最終日曜日に始まり10月最終日曜日に終了します。

どの国が夏時間を実施していますか？

北米、欧州、中東の一部、オーストラリア南部、ニュージーランドなど、世界約70か国が実施しています。赤道付近の熱帯諸国やアジアの大半の国は実施していません。

夏時間はUTCオフセットを変更しますか？

はい。夏時間が開始されると、その地域の公式UTCオフセットが1時間進みます（例：米東部標準時UTC-5は夏期に東部夏時間UTC-4へと変わります）。

なぜ夏時間によって都市間の時差が変化するのですか？

国や地域によって夏時間への移行日程が異なるため、春と秋の切り替え期間中は2都市間の時差が1時間広がったり縮まったりします。

すべての国が夏時間を採用しているのですか？

いいえ。世界人口の大多数は夏時間を使用していません。特に赤道付近の熱帯国では年間を通じて昼夜の長さがほぼ一定であるため、時計を変更する必要がありません。