When is daylight saving time?

1.1 Overview and General Schedule

The primary mechanism of daylight saving time involves shifting standard time forward by sixty minutes during the spring season and shifting it back by the same amount in the fall. A common mnemonic used to remember the direction of the clock adjustment is "spring forward, fall back."^{[1]영어권에서는 'Spring forward, fall back(봄에는 앞으로, 가을에는 뒤로)'이라는 말장난 같은 문구로 시계 바늘을 돌리는 방향을 외운다.}

Because the natural duration of daylight varies according to latitude and season, daylight saving time is predominantly utilized in temperate regions. Tropical regions near the Equator generally do not observe this system, as the variation in daylight hours throughout the year is negligible. Similarly, high-latitude polar regions experience extreme variations in daylight, rendering the one-hour shift functionally impractical.

The specific dates for the start and end of daylight saving time are typically scheduled on weekends—most commonly early Sunday mornings—to minimize disruption to commercial activities, transportation networks, and daily routines.

1.2 Observance by Geographic Region

1.3 Legislative and Global Variations

The implementation dates and status of daylight saving time remain subject to continuous legislative revision globally. National governments periodically evaluate the system based on economic impacts, energy consumption studies, public health outcomes, and societal preferences. Consequently, various countries have permanently adopted year-round standard time or year-round daylight saving time, while others frequently alter the specific dates of transition.

2.1 Temporal Asynchrony of the Northern Hemisphere

While the majority of countries implementing Daylight Saving Time (DST) reside in the Northern Hemisphere, the precise dates of transition vary significantly by region. In North America (including the United States and Canada), DST currently commences on the second Sunday in March and terminates on the first Sunday in November. Conversely, the European Union initiates the transition on the last Sunday in March and reverts to standard time on the last Sunday in October.

This creates a two-to-three-week temporal discrepancy twice a year—known as "phase lags"—where the standard time difference between transatlantic locations (such as London and New York) narrows from five hours to four hours, causing substantial logistical friction in international finance, aviation, and broadcasting schedules.

2.2 Equatorial Invalidation and Tropical Invariance

The efficacy of DST relies on the geometric variance of axial tilt, which dictates the proportion of sunlight hours relative to latitude. Near the Equator, day length remains virtually constant at 12 hours throughout the year. Consequently, equatorial nations (such as Ecuador, Kenya, and Indonesia) do not observe DST, as shifting the clock provides zero gain in usable daylight hours.

The application of DST becomes progressively more pronounced toward higher latitudes; however, extreme polar regions often forego the system as well, since shifting an hour is trivial when experiencing 24-hour polar day or midnight sun.

2.3 Southern Hemisphere Inversion

For nations in the Southern Hemisphere that observe DST (such as parts of Australia, New Zealand, and Chile), the seasonal calendar is completely inverted. DST begins during their spring (typically October or November) and concludes during their autumn (typically March or April).

This inversion results in anomalous period alignments. For instance, when Sydney shifts its clocks forward to enter DST in October, and London shifts its clocks back to exit DST in late October, the time difference between the two cities changes abruptly by two full hours within a single month.

2.4 The Spring Hour Loss and Statistical Anomalies

The transition into DST—specifically the "spring forward" phase where a 60-minute duration is excised overnight—acts as an unintended global chronobiology experiment. Epidemiological studies demonstrate a statistically significant surge in acute myocardial infarctions (heart attacks), workplace injuries, and fatal motor vehicle accidents during the immediate Monday and Tuesday following the spring transition.^{[4]https://www.nejm.org/doi/full/10.1056/NEJMc0807104 — 2008년 뉴잉글랜드 의학저널(NEJM)에 발표된 연구에 따르면 봄철 시계 바늘을 한 시간 앞당긴 직후 월요일에 심근경색 발생률이 통계적으로 유의미하게 증가한다.}

This phenomenon is primarily attributed to acute sleep deprivation and systemic circadian desynchronization across large populations. Conversely, the "fall back" transition exhibits a transient decrease in cardiac events.

2.5 History of American Schedule Expansions

The dates for DST in the United States have undergone multiple statutory expansions driven by economic lobbying rather than astronomical shifts. Under the *Uniform Time Act of 1966*, DST ran from the last Sunday in April to the last Sunday in October.

Congress subsequently expanded this framework: first in 1986 (moving the start to the first Sunday in April), and most recently via the *Energy Policy Act of 2005* (effective 2007), which pushed the start date to March and the end date to November. The extension into November was heavily lobbied by the retail and confectionery industries to ensure that children could trick-or-treat under daylight on Halloween.^{[5]https://www.congress.gov/bill/109th-congress/house-bill/6 — 미국 제과협회 등 로비 단체들은 어린이들이 할로윈 데이 밤에 더 안전하고 오래 사탕을 모으러 다닐 수 있도록 10월 말 대신 11월 첫째 주 일요일로 DST 종료 일을 미루도록 꾸준히 압박했다.}

2.6 The Emergency Energy Conservation Act of 1974

In response to the 1973 OPEC oil embargo, United States President Richard Nixon signed the *Emergency Daylight Saving Time Energy Conservation Act*, ordering year-round DST starting January 6, 1974. The goal was to reduce peak electricity consumption.

However, the experiment generated intense public backlash. During the midwinter months of January and February, sunrise occurred as late as 8:00 AM or 8:30 AM in many regions, forcing school children to commute in complete darkness. Following a series of fatal traffic accidents involving pedestrians, public approval plunged, prompting Congress to repeal the year-round mandate prematurely in October 1974.

2.7 Midnight Boundary Complications in Law

The precise moment of clock transition—typically set at 02:00 local time—was engineered specifically to minimize disruptions to daily commerce and transportation schedules. However, it creates unique legal paradoxes regarding birth and death certificates.

During the fall transition, the hour between 01:00 AM and 02:00 AM occurs twice. If Twin A is born at 01:45 AM, and Twin B is born 30 minutes later after the clocks drop back one hour, Twin B’s official time of birth will be logged as 01:15 AM.^{[6]가을철 시계 바늘을 한 시간 뒤로 돌리는 시점에 쌍둥이가 태어나면 나중에 태어난 동생의 출생 기록 시간이 형보다 앞서게 되는 법적 시간 서열의 역전 현상이 실제로 발생한다.} On paper, Twin B becomes chronologically "older" than Twin A despite being delivered second.