When do clocks go forward?

1.1 Overview

The practice of advancing clocks by one hour, often described colloquially as "springing forward," marks the beginning of Daylight Saving Time. During this transition, local time shifts forward instantaneously—typically from 2:00 AM to 3:00 AM—effectively shortening the day of the transition to twenty-three hours.^{[1]새벽 2시에서 3시로 시간이 훌쩍 뛰어넘어 가버리기 때문에 이 날은 하루가 물리적으로 23시간이 된다. 비행기나 기차 운행표가 꼬이는 원인이기도 하다.} The primary objective is to align human activity patterns with seasonal shifts in daylight, transferring an hour of daylight from the morning to the evening.

In most observing regions in the Northern Hemisphere, this change occurs between March and April. Conversely, in the Southern Hemisphere, where the seasons are inverted, clocks are moved forward between September and October. The process is reversed in the autumn season, commonly referred to as "falling back," when clocks are set back by one hour to return to standard time.

1.2 Regional Practices

The precise dates and rules governing when clocks go forward vary depending on national legislation and geographic location.

In North America, including the majority of the United States and Canada, the shift takes place on the second Sunday of March.^{[2]https://www.transportation.gov/regulations/daylight-saving-time — 미국은 2007년부터 에너지 정책법(Energy Policy Act of 2005)에 따라 3월 둘째 주 일요일부터 11월 첫째 주 일요일까지 서머타임을 적용하고 있다.} Automated systems, smartphones, and network-connected devices perform this adjustment automatically at 2:00 AM local time, while manual timekeeping devices require human intervention.

In the European Union and participating European nations, the transition is standardized across time zones to maintain synchronization. Clocks advance on the last Sunday of March. The change occurs simultaneously across the continent based on Coordinated Universal Time (UTC), meaning the local time of the shift varies depending on the specific European time zone.

In the Southern Hemisphere, regions such as parts of Australia, New Zealand, and select South American countries adjust their clocks forward during their spring, which falls in the latter half of the calendar year.

A significant portion of the world's population does not observe this practice. Most countries near the Equator maintain standard time year-round, as seasonal fluctuations in daylight hours are negligible. Additionally, several major nations that previously observed daylight adjustments have permanently discontinued the practice.

1.3 Impact and Controversy

The transition of moving clocks forward carries several notable impacts on modern society:

• Energy Consumption: Originally conceived as a measure to reduce artificial lighting fuel consumption, modern studies show conflicting results regarding its contemporary energy-saving efficiency due to increased usage of heating and air conditioning.
• Public Health: The immediate loss of one hour of sleep during the spring transition is linked to temporary disruptions in circadian rhythms. Studies observe short-term increases in workplace fatigue, traffic accidents, and cardiovascular events immediately following the time change.
• Economic Effects: Industries related to outdoor recreation, retail, and tourism generally experience increased revenue due to extended evening daylight. Conversely, the aviation and logistics sectors face operational complexities when coordinating international schedules across regions with non-uniform transition dates.

Due to these factors, public and political debates regarding the permanent abolition of biannual clock shifts remain active in several jurisdictions, with proposals favoring either permanent Daylight Saving Time or permanent standard time.

2.1 William Willett and the British Golf Connection

The modern iteration of Daylight Saving Time (DST) owes its existence to William Willett, a British builder and avid golfer. In 1907, frustrated by his golf games being cut short by dusk, Willett self-published a pamphlet titled *The Waste of Daylight*.^{[3]https://www.parliament.uk/about/living-heritage/transformingsociety/towncountry/landscape/overview/daylightsaving — 앤디 머리의 조상쯤 되는 영국의 건축업자 윌리엄 윌렛은 골프를 더 치고 싶은 마음에 이 사재를 털어 팸플릿을 집필했다.} He proposed advancing clocks by 80 minutes in four 20-minute increments during April and reversing the process in September. Although Willett spent his fortune lobbying the British Parliament, he died in 1915 without seeing his idea implemented. The United Kingdom officially adopted British Summer Time (BST) a year after his death, in 1916.

2.2 Imperial Germany's Wartime Energy Strategy

While New Zealand entomologist George Hudson proposed a two-hour shift in 1895 to collect insects, the German Empire became the first nation to implement DST on a national scale. On April 30, 1916, amidst World War I, Germany and its ally Austria-Hungary advanced all clocks by one hour.^{[4]https://www.history.com/news/8-things-you-may-not-know-about-daylight-saving-time — 평화로운 일상 개선이 아니라 제1차 세계 대전이라는 참혹한 전쟁 상황 속에서 석탄 절약을 위해 최초로 전격 도입되었다.} The measure was designed to conserve artificial lighting fuel—specifically coal—for the war effort. The United Kingdom and several other European nations followed suit within weeks, rendering DST an instrument of wartime geopolitics rather than public lifestyle optimization.

2.3 The Spring-Forward Sleep Deprivation Phenomenon

The loss of a single hour during the spring transition generates measurable physiological stress across human populations. Chronobiologists refer to the phenomenon as "social jetlag." Epidemiological studies indicate a statistically significant spike in adverse events on the Monday immediately following the spring shift:

• Cardiovascular Events: Studies published in the *Open Heart* journal indicate a 24% increase in reported acute myocardial infarctions (heart attacks) on the Monday following the clock change.
• Occupational Safety: Data from the American Psychological Association demonstrates a sharp increase in the severity of workplace injuries occurring on the first workday of DST.
• Automotive Casualties: Traffic safety analyses in North America report a 6% increase in fatal motor vehicle collisions during the six days following the spring clock change, attributed to collective circadian disruption.

2.4 Arizona and Indiana: Regional Non-Conformity

Within the United States, time policy remains decentralized under the Uniform Time Act of 1966, allowing individual states to opt out of DST.

• The Arizona Exception: Arizona abolished DST in 1968. The decision stems from the state's extreme summer heat; advancing clocks would push daylight further into the evening, increasing residential air conditioning costs and peak energy demand. However, the Navajo Nation inside northern Arizona continues to observe DST to align with its territories spanning into Utah and New Mexico, creating an enclave effect where the Hopi Reservation—entirely surrounded by the Navajo Nation—does not observe DST.^{[5]애리조나주 안에 나바호 국수가 있고, 그 나바호 국수 안에 호피 예약구가 마트료시카처럼 겹겹이 위치해 있어 도로를 달리다 보면 서머타임 영역이 계속 바뀌는 기현상이 발생한다.}
• The Indiana Split: Until 2006, the majority of Indiana did not observe DST, resulting in decades of logistical disruption for interstate commerce. The state eventually standardized DST observation under legislative pressure to align its economy with surrounding metropolitan hubs like Chicago and Louisville.

2.5 The Sunshine Protection Act and Perpetual DST Debate

The debate surrounding the permanent status of DST has shifted from seasonal shifts to structural elimination. In the United States, the federal *Sunshine Protection Act* was passed unanimously by the Senate in March 2022. The legislation aimed to make Daylight Saving Time permanent nationwide, effectively eliminating the requirement to set clocks back in November.

However, the bill stalled in the House of Representatives due to opposition from public health experts and chronobiologists. The American Academy of Sleep Medicine argues that permanent *Standard Time*—not permanent DST—aligns far better with the natural human circadian rhythm. Under permanent DST, northern regions would experience daylight as late as 9:00 AM during mid-winter, causing prolonged morning darkness during school and commute hours.

2.6 Digital Systems and the POSIX Epoch Timestamp

Modern computing architectures do not natively recognize human concepts of time zones or DST shifts. Operating systems running on the POSIX standard measure time as the continuous count of seconds elapsed since 00:00:00 UTC on January 1, 1970 (the Unix Epoch).

The shift of local clocks forward is managed by system-level software using localized time zone databases, maintained by the Internet Assigned Numbers Authority (IANA). When a device shifts its display clock forward from 01:59:59 to 03:00:00, the underlying epoch timestamp continues uninterrupted. Consequently, software bugs frequently arise during spring transitions when scheduled background processes (cron jobs) set to run between 02:00 and 03:00 are skipped entirely by logic engines expecting standard hourly progression.^{[6]새벽 2시 15분에 실행되도록 설정된 데이터베이스 백업 작업 등은 2시 자체가 존재하지 않고 3시로 튀어버리기 때문에 당일 아예 실행되지 않고 누락되는 사고가 터지곤 한다.}