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In modern usage, civil time refers to statutory time as designated by civilian authorities. Modern civil time is generally national standard time in a time zone at a fixed offset from Coordinated Universal Time (UTC), possibly adjusted by daylight saving time during part of the year. UTC is calculated by reference to atomic clocks and was adopted in 1972.[1] Older systems use telescope observations.

In traditional astronomical usage, civil time was mean solar time reckoned from midnight. Before 1925, the astronomical time 00:00:00 meant noon, twelve hours after the civil time 00:00:00 which meant midnight. HM Nautical Almanac Office in the United Kingdom used Greenwich Mean Time (GMT) for both conventions, leading to ambiguity[clarification needed], whereas the Nautical Almanac Office at the United States Naval Observatory used GMT for the pre-1925 convention and Greenwich Civil Time (GCT) for the post-1924 convention until 1952. In 1928, the International Astronomical Union introduced the term Universal Time for GMT beginning at midnight.[2]

In modern usage, GMT is no longer a formal standard reference time: it is now a name for the time zone UTC+00:00. Universal Time is now determined by reference to distant celestial objects: UTC is derived from International Atomic Time (TAI), and is adjusted by leap seconds to compensate for variations in the rotational velocity of the Earth.[3] Civil Times around the world are all defined by reference to UTC. In many jurisdictions, legislation has not been updated and still refers to GMT; this is taken to mean UTC+0.

History

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The division of the day into times of day has existed since the beginning of the calendar.

Twelve hours: Babylonian and Roman division of the day

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People in Antiquity divided the day into twelve hours, but these were reckoned from sunrise rather than midnight. Babylonian hours were of equal length, while Roman temporal hours varied depending on the season.

The Horae, literally "the hours," were the original Greek goddesses who oversaw regulated life. They were the patron goddesses of the various times of day. In Greek tradition, the twelve hours were counted from just before sunrise to just after sunset.

Roman daytimes were called hora (hours), with the morning hour as hora prima. The night was divided into four sections called vigilia (night watch), two before midnight and two after.[4] The Romans originally counted the morning hours backwards: "3 a. m." or "3 hours ante meridiem" meant "three hours before noon", in contrast to the modern meaning "three hours after midnight".

This ancient division has survived in the Liturgy of the Hours: Prime, Terce, Sext, and Nones are named after the first, third, sixth and ninth hours of the day. The Matins, the nocturnal prayer is, according to the Rule of Saint Benedict, to be prayed at the "eighth hour of the night", which corresponds to about 2 am.

The Spanish siesta derives its name from the Latin hora sexta for the sixth hour (noon).

Middle East

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In Semitic language cultures, the day traditionally begins at nightfall. This is still important today for the beginning of Shabbat and Islamic holidays.

A division of days has survived from Persian, following the Babylonian beginning of the day: The rōsgār (times of day) are hāwan (morning), uapihwin (afternoon), usērin (evening), ēbsrūsrim (sunset to midnight), and ushahin (midnight to dawn). The last two are collectively called shab (night).

Middle Ages and early modern times

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The modern division of the day into twenty-four hours of equal length (Italian hours) first appeared in the 14th century with the invention of the mechanical wheel clock and its widespread use in turret clocks.

With the onset of industrialization, working hours became tied to the clock rather than to daylight.[5]

See also

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References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Civil time

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Civil time is the standardized system of timekeeping employed in daily civil activities, commerce, and governance, which approximates the mean solar day divided into 24 equal hours starting at and adjusted into global time zones for uniformity. It serves as the basis for coordinating schedules, transportation, and legal matters, distinguishing it from astronomical time systems that track celestial events more precisely. Unlike apparent , which varies due to Earth's elliptical orbit and , civil time relies on mean —a uniform average of the solar day—to ensure consistency. The foundation of civil time lies in the concept of mean solar time, which assumes the Sun travels across the sky at a constant speed, ignoring irregularities captured by the equation of time that can cause discrepancies of up to 16 minutes between readings and clock time. This equation arises from two main factors: the obliquity of the , which tilts Earth's rotational axis relative to its , and the eccentricity of Earth's orbit around the Sun. Civil time zones, typically spanning 15 degrees of to align with the , center on standard meridians where matches the zone's official time; deviations occur farther from these meridians, amounting to about four minutes per degree of . In practice, boundaries are often adjusted for political, geographic, or economic reasons rather than strict longitudinal lines. Historically, the need for civil time standardization emerged in the 19th century amid rapid advancements in railroads and telegraphy, which rendered hundreds of local solar times impractical for scheduling. In the United States, railroads adopted four main time zones on November 18, 1883, reducing chaos from over 300 local times, a change later codified by the 1918 Standard Time Act. Globally, the 1884 International Meridian Conference in Washington, D.C., established the Greenwich Meridian as the prime reference and promoted Greenwich Mean Time (GMT) as the international standard, facilitating worldwide adoption of hourly time zones by the early 20th century. Daylight saving time, first widely implemented during World War I, further modifies civil time seasonally in many regions to extend evening daylight. In the modern era, civil time is anchored to (UTC), an atomic time scale maintained by international atomic clocks and synchronized with through occasional leap seconds to stay within 0.9 seconds of (UT1). UTC replaced GMT as the global civil standard in 1972, ensuring high precision for applications like GPS, , and . While most countries observe UTC offsets in whole hours, exceptions include half-hour or quarter-hour variations in places like , , and Newfoundland, reflecting ongoing adaptations to local needs.

Definition and Basis

Definition of Civil Time

Civil time is the official, standardized time established by governments and legal authorities for civil purposes, including business operations, transportation schedules, public services, and everyday societal activities. It serves as the legally recognized time scale in a given locality or , ensuring uniformity and coordination among individuals and institutions. This time system is fundamentally based on mean , adjusted for the specific of the region or , and is maintained through atomic clocks synchronized to (UTC), which incorporates occasional leap seconds to align closely with Earth's rotation. Unlike apparent solar time, which tracks the actual position of the Sun across the and varies irregularly due to Earth's elliptical orbit and , civil time prioritizes consistency by employing a uniform average (mean) solar day. Apparent solar time, as read from a , can deviate from mean solar time by up to 16 minutes throughout the year, making it impractical for modern scheduling. Similarly, civil time differs from , a star-based system used primarily in astronomy that measures relative to distant stars; a sidereal day lasts about 23 hours, 56 minutes, and 4 seconds, reflecting the planet's daily spin without accounting for its orbital motion around the Sun. Civil time's emphasis on uniformity over precise celestial alignment supports reliable, predictable daily routines. The structure of the civil day is a 24-hour period divided into 24 equal hours, each comprising 60 minutes of 60 seconds, beginning at —defined as 12 hours after local mean solar noon—to facilitate a nocturnal-to-diurnal cycle aligned with human activity patterns. This midnight start contrasts with traditional astronomical conventions where days often begin at noon, but it has become the global standard for civil reckoning. In practice, civil time synchronizes analog and digital clocks in homes, schools, workplaces, and transportation hubs within the same , such as all locations in the observing the same hour regardless of minor differences. Time zones extend this standardization across broader geographic areas to approximate while minimizing disruptions.

Mean Solar Time and the Equation of Time

Mean is the time scale derived from the average length of a solar day over the course of a year, assuming a uniform 24-hour day based on around the Sun. This contrasts with apparent , which is measured directly by the position of the Sun in the sky and varies slightly due to irregularities in Earth's motion. The mean solar day is precisely seconds long, providing a consistent basis for timekeeping that smooths out these natural variations. The equation of time quantifies the discrepancy between apparent and mean , arising primarily from two astronomical factors: Earth's elliptical orbit, which causes the planet to move at varying speeds relative to the Sun (faster near perihelion and slower near aphelion, per Kepler's second law), and the 23.44° , which affects the Sun's apparent path and leads to an obliquity component in the timing. Mathematically, it is defined as E=apparent solar time−mean solar time,E = \text{apparent solar time} - \text{mean solar time}, where EE represents the correction needed to convert between the two scales; a positive value indicates that apparent time is ahead of mean time (e.g., the Sun reaches its highest point before clock noon), while a negative value means it lags behind. This difference never exceeds about 20 minutes in magnitude but is crucial for understanding why sundials do not align perfectly with standard clocks. Over the year, the equation of time exhibits a characteristic annual variation, tracing a figure-eight pattern known as the when plotted against the Sun's position. It reaches its most negative value of approximately -14 minutes around February 11, when solar time is ahead of apparent time, and its most positive value of about +16 minutes near , when apparent time leads time. Additional crossings occur around April 15, June 13, September 1, and December 25, where the two times coincide (E = 0). These fluctuations result from the combined effects of and , with the elliptical orbit dominating the overall shape and the tilt introducing asymmetry. In practice, civil time approximates mean to provide a standardized, uniform measure for societal use, disregarding the equation of time's minor variations which are irrelevant for most daily activities. This approximation ensures clocks maintain a steady 24-hour cycle, with local mean solar time serving as the reference at each before further adjustments like time zones.

Historical Evolution

Ancient Timekeeping

The earliest human efforts to measure time for civil purposes relied on observing natural cycles, such as the positions of the sun, , and stars, to divide the day into basic periods for activities like and gathering. Prehistoric peoples, starting around years ago, tracked lunar phases to mark longer intervals, while daily divisions were gauged by the sun's shadow length and apparent motion across the sky. In , timekeeping advanced with the development of observational tools during the New Kingdom around 1500 BCE, including shadow clocks and water clocks known as clepsydrae. The shadow clock, a portable T-shaped device, used a crossbar to cast shadows on marked lines, dividing daylight into 12 hours plus twilight periods, oriented east-west and rotated at noon. Water clocks measured time by the steady flow of water from a container, useful for nighttime or overcast conditions when sundials failed. These innovations built on earlier solar observations tied to the Nile's annual flood cycles. Babylonians, drawing from Sumerian traditions around 2000 BCE, introduced a (base-60) system that profoundly influenced time divisions, subdividing units into 60 parts for precision in astronomy and daily life. They divided the full day—sunrise to sunrise—into 12 double-hours called bēru, each roughly equivalent to two modern hours, further broken into 30 uṣ (about 4 minutes) and 60 ninda (about 4 seconds), reflecting their positional . This structure, rooted in celestial tracking, laid the foundation for the 60-minute hour still used today. Greek and Roman societies adapted these earlier systems, formalizing a 12-hour cycle for daylight and another for night, with hours varying seasonally to equalize the 12 parts regardless of day length—shorter in winter, longer in summer. In , the hora initially denoted these variable seasonal intervals, measured by sundials or water clocks, transitioning gradually toward more consistent divisions influenced by Greek astronomical refinements around 300 BCE. Key artifacts illustrate these developments: the Egyptian shadow clock from the reign of (c. 1500 BCE), an L-shaped stone aligning with the rising sun to mark hours for rituals; and numerous Roman sundials, such as the well-preserved inscribed bronze example from (1st century BCE or earlier), often portable and shaped like everyday objects for practical use. These ancient methods held deep cultural significance, synchronizing civil life with natural rhythms essential for —such as timing planting with Nile floods in or seasonal star risings in —and religious practices, including temple rituals and festivals aligned with solar and lunar events like the of Sirius (). In , sundials in public forums and private villas symbolized civic order and elite status, reinforcing communal observances tied to agrarian cycles and divine calendars.

Medieval Developments

During the , Islamic scholars advanced timekeeping technologies that significantly influenced developments. In the Islamic world, sophisticated water clocks and astrolabes were refined for precise astronomical observations and daily prayer timings, with notable contributions from engineers like . Al-Jazari's 1206 work, The Book of Knowledge of Ingenious Mechanical Devices, described innovative automata and a monumental clock in that used water power to simulate celestial movements and display time through programmable elephant figures, marking one of the earliest known analog computing devices. These inventions spread to through trade routes and scholarly translations, inspiring later mechanical designs. In , the invention of the mechanical clock emerged in the early , primarily driven by the need for accurate timekeeping in monasteries to regulate the for prayers, work, and rest. The key innovation was the mechanism, which allowed for consistent oscillation of a weighted foliot, providing the first reliable way to measure equal intervals without relying on water or sand flow. Early examples appeared around 1300 in Italian and English monastic settings, such as the clock built by Richard of Wallingford at St. Albans Abbey circa 1326–1334, which integrated astronomical functions. This shift marked a departure from earlier analog methods, enabling more precise civil and ecclesiastical scheduling. The adoption of mechanical clocks facilitated a crucial transition from variable seasonal hours—where daylight was divided into 12 unequal parts that lengthened in summer and shortened in winter—to fixed equinoctial hours of equal length, aligning with the 24-hour day. This change was propelled by the demand for standardized times across regions and the growth of urban centers, where consistent time supported and . By the mid-14th century, began appearing in public spaces, such as those in and , promoting communal awareness of time. Key milestones included the installation of the first public clock in in 1336, documented by Dominican Galvano Fiamma as a mechanical device on the Torre dei Borri with bells to announce hours. Similarly, in 1370, King Charles V of France commissioned the first public clock for the in , crafted by Henri de Vic, featuring a dial visible to the public. These public installations extended timekeeping's role beyond religious use, influencing civil life by regulating market openings, curfews, and activities, thus fostering economic coordination in burgeoning medieval cities.

Standardization in the Modern Era

In the 19th century, the rapid expansion of railroads across Britain and created significant challenges for timekeeping, as each locality adhered to its own , leading to discrepancies of up to several minutes between nearby towns. This chaos resulted in scheduling errors, safety risks, and inefficiencies for rail operations spanning hundreds of miles. To address these issues, the Great Western Railway in Britain became the first to implement a standardized "railway time" based on time in 1840, synchronizing clocks along its network using telegraph signals. By 1847, most British railway companies had adopted this single , marking an early step toward national uniformity driven by industrial needs. In the United States, similar problems prompted the rail industry to act independently. On , 1883—known as the "Day of Two Noons"—American and Canadian railroads simultaneously adopted four continental time zones (Eastern, Central, Mountain, and Pacific), replacing over 100 local times with standardized meridians set 15 degrees apart. This voluntary system, coordinated by the General Time Convention, was influenced by Canadian engineer , who, after missing a train due to time confusion in 1876, advocated for global time zones in his 1879 pamphlet Time-Reckoning for All Nations. Fleming proposed dividing the world into 24 zones based on the Greenwich meridian, a concept that gained international traction. The push for global standardization culminated in the held in , from October 1 to 22, 1884, attended by delegates from 25 nations. The conference unanimously adopted the Greenwich meridian as the for and recommended a universal day beginning at , while proposing an initial framework of 24 zones offset by one hour each. Although not immediately binding, these resolutions provided a foundation for worldwide adoption, with Fleming serving as a key Canadian representative. The 20th century saw further refinements amid global conflicts and technological advances. During , the need for coordinated logistics accelerated implementation; in the United States, the of 1918 (also known as the Calder Act) federally mandated the four time zones established by railroads, assigning regulatory authority to the . further promoted uniformity, as nations aligned civil time for military and economic efficiency, leading to near-universal adoption of time zones by the mid-century. Post-, civil time systems began aligning with precursors to (UTC), including the International Time Bureau's efforts starting in the 1920s to synchronize astronomical observations with mean , culminating in UTC's formal introduction in 1960 as a basis for international civil timekeeping. This evolution ensured civil time's precision for , , and global trade.

Time Zones

Origins of Time Zones

Prior to the establishment of time zones, civil time in the was determined locally based on the sun's position, resulting in significant inconsistencies across regions. , there were approximately 100 different local times by 1883, while as a whole had over 144 variations, leading to scheduling chaos for expanding railroad networks and telegraph systems. In , similar fragmentation occurred, with each community or city maintaining its own solar-based time, complicating cross-border rail travel and communication as networks grew rapidly in the mid-1800s. The push for standardization began with railroad initiatives to resolve these practical issues. In Britain, the recommended in 1847 that all railway companies adopt (GMT) as a uniform standard for scheduling, marking an early step toward coordinated civil time across the network. This was followed by a more comprehensive proposal in : on November 18, 1883, U.S. and Canadian railroads simultaneously implemented four standard time zones—Eastern, Central, , and Pacific—each 15 degrees of wide and one hour apart, effectively replacing the myriad local times for rail operations. The adoption of time zones spread globally but faced resistance from communities attached to local solar time. Canada integrated the 1883 railroad zones into broader use shortly thereafter, while European countries gradually followed suit in the late 19th and early 20th centuries, often aligning with GMT offsets for rail and maritime coordination. Notable resistance persisted in places like Detroit, Michigan, which clung to its local mean time—28 minutes behind Eastern Standard Time—until a 1905 citywide vote aligned it with the Central zone to accommodate rail schedules. A pivotal milestone came at the 1884 International Meridian Conference in Washington, D.C., where 25 nations agreed on Greenwich as the prime meridian and a universal day based on mean solar time, laying the groundwork for worldwide time zone alignment without directly mandating zones. By the early 1900s, legal enforcement emerged in various countries, such as Germany's 1893 standardization to a single zone and the U.S. Congress's 1918 Standard Time Act, which formalized the railroad zones under federal oversight.

Current Time Zone System

The current time zone system divides the world into approximately 38 distinct zones, departing from the ideal 24 primary zones that would align with 15-degree intervals for one-hour offsets from (UTC). Each primary zone theoretically spans 15 degrees of , corresponding to one hour of difference, with offsets ranging from UTC-12 to UTC+14; for instance, Eastern Standard Time in observes UTC-5. This framework emerged from the 19th-century efforts but has evolved to accommodate practical needs, resulting in non-standard offsets such as half-hour and quarter-hour variations. Boundary irregularities frequently override strict longitudinal divisions due to political, economic, and administrative considerations, leading to the effective total of about 40 zones in practice. For example, , which spans five geographical time zones, maintains a single nationwide zone at UTC+8 for national unity, while utilizes 11 zones across its vast territory to better match local . Similarly, adheres to UTC+5:30, a half-hour offset that simplifies and across its subcontinent. These deviations ensure that time zones often follow borders, state lines, or even municipal decisions rather than pure geography. The legal foundation of the system rests on national and subnational laws, which define offsets, boundaries, and transitions within each jurisdiction, compiled and standardized for global use by the (IANA) time zone database. Established under Best Current Practice 175 (BCP 175) and updated periodically—most recently in version 2025b—this public-domain database records historical and projected civil time data for representative locations worldwide, enabling accurate computation in software and devices. It serves as the authoritative reference for developers and systems to handle offsets and rules without relying on proprietary sources. In , the system features four contiguous zones for the and : Eastern Time (UTC-5 standard), Central Time (UTC-6), Mountain Time (UTC-7), and Pacific Time (UTC-8), with additional zones like (UTC-9) and Hawaii-Aleutian (UTC-10) for outlying areas. These zones facilitate coordination in a densely interconnected , though their misalignment with political boundaries can complicate cross-border activities. The overall framework profoundly influences international travel, where abrupt offset changes affect flight schedules and calculations, and , where precise prevents errors in global applications like financial transactions and .

Adjustments to Civil Time

Daylight Saving Time

Daylight saving time (DST) is a to civil time in which clocks are advanced by one hour, typically during warmer months, to make better use of evening daylight and align daily activities with patterns. The primary purpose is to conserve by reducing the need for artificial in the evenings, though this benefit has been widely debated. Originating as a practical idea, DST shifts the standard time forward in spring and back in autumn, extending daylight into later hours without altering sunrise times significantly. The concept traces back to a 1784 satirical essay by , who humorously suggested Parisians wake earlier to save on candle wax, though it was not a serious proposal for clock changes. It was formalized in 1907 by British builder , who advocated advancing clocks by 80 minutes in incremental steps during summer to promote and energy efficiency. Germany became the first country to implement DST on April 30, 1916, during , aiming to conserve coal for the by minimizing evening energy use. The practice spread rapidly to allies and other nations during the war, and was revived in for similar resource-saving reasons. In the United States, the of 1966 standardized DST nationwide, setting uniform start and end dates while allowing exemptions for states or territories. Today, DST is observed in about 70 countries, primarily in and , with variations in duration and dates. The harmonizes its observance, beginning on the last Sunday in March and ending on the last Sunday in October, to facilitate cross-border coordination. In contrast, most countries in and do not observe DST due to minimal seasonal daylight variations near the and cultural or economic factors. For example, major economies like , , and have never adopted it, while some nations like and abolished DST in recent years to simplify timekeeping. The effects of DST remain contentious, with initial energy savings claims from wartime implementations now disputed by modern studies. One analysis found that DST actually increases residential demand by about 1% overall, due to higher use outweighing reductions in some regions. Health impacts include disrupted circadian rhythms from clock changes, leading to short-term increases in , , and a modest rise in acute risk following the spring transition. Economically, proponents argue it boosts retail and sectors by extending evening hours, but critics highlight costs from scheduling disruptions and accidents. Debates have intensified recently, with several U.S. states like advocating for permanent DST to eliminate biannual changes, and the EU Parliament voting in 2019 to end the practice—though implementation stalled due to lack of consensus on permanent .

Relation to UTC and Leap Seconds

Coordinated Universal Time (UTC) serves as the primary international standard for civil timekeeping, forming the foundation for all global time zones. It is computed by the International Bureau of Weights and Measures (BIPM) based on (TAI), which is derived from a weighted average of over 400 atomic clocks maintained by institutions worldwide, ensuring high precision independent of astronomical observations. To align UTC with Earth's rotation—approximating mean —leap seconds are periodically inserted, keeping UTC within 0.9 seconds of 1 (UT1), the irregular solar-based scale. Civil time in each is defined by a fixed offset from UTC, typically in whole hours, such as UTC+0 for (now synonymous with UTC in standard usage) or UTC-5 for Eastern Standard Time. These offsets allow of clocks for legal, commercial, and social purposes while disregarding UTC's sub-second atomic precision, as civil time is generally maintained to the nearest second on analog or digital displays. This alignment ensures that civil time remains practical and uniform, with UTC providing the atomic backbone for global coordination without requiring adjustments for local solar variations beyond time zone boundaries. Leap seconds are irregular adjustments added to UTC—typically at the end of or December—to account for the gradual slowing of due to tidal friction and other geophysical effects, preventing divergence from . Since the introduction of UTC on January 1, 1972, a total of 27 positive s have been inserted, with the most recent on December 31, 2016; no negative leap seconds (subtractions) have occurred to date. The decision to introduce a leap second is made by the International Earth Rotation and Reference Systems Service (IERS), which monitors using and other techniques, announcing the change approximately six months in advance to relevant international bodies like the BIPM and the (ITU). This process maintains UTC's dual role as both an atomic and astronomical , though insertions are infrequent and unpredictable. Ongoing debates center on the challenges leap seconds pose to digital systems, such as software glitches during insertions that affect , financial trading, and computing networks, prompting calls for stability in an increasingly atomic-time-dependent world. In November 2022, the General Conference on Weights and Measures (CGPM) adopted Resolution 4, approving the cessation of s by 2035 to allow UTC to drift gradually from UT1 without further adjustments, potentially for a century or more, while preserving the scale for scientific and navigational needs. This decision, supported by major stakeholders including the ITU and BIPM, aims to eliminate disruptions while monitoring whether might necessitate earlier intervention, such as a rare negative leap second.

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  74. https://www.nist.gov/pml/time-and-frequency-division/time-realization/leap-seconds
  75. https://www.timeanddate.com/time/leapseconds.html
  76. https://www.bipm.org/documents/20126/28429869/working-document-ID-7399/aed6f662-7a8a-64b3-3b70-f36d3c8ef037
  77. https://www.nature.com/articles/d41586-022-03783-5
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