Time Zone

Jai Ganesh · Yesterday 21:56:39

Time Zone

Gist

Time zones divide the world into 24 segments, each about 15 degrees of longitude wide, to standardize time, as the Earth rotates 15 degrees every hour. They create uniform local times, offset from Coordinated Universal Time (UTC), allowing for synchronized schedules despite different sunrises/sunsets, with political boundaries often adjusting ideal 15-degree lines for practicality and some regions using Daylight Saving Time (DST).

Time zones are primarily based on the Earth's rotation and its division into 24 longitudinal sections, with each representing one hour of the day. The prime meridian, located at Greenwich, England (Greenwich Mean Time or GMT), serves as the starting point for defining time zones.

Summary

A time zone is an area which observes a uniform standard time for legal, commercial, and social purposes. Time zones tend to follow the boundaries between countries and their subdivisions instead of strictly following longitude, because it is convenient for areas in frequent communication to keep the same time.

Each time zone is defined by a standard offset from Coordinated Universal Time (UTC). The offsets range from UTC−12:00 to UTC+14:00, and are usually a whole number of hours, but a few zones are offset by an additional 30 or 45 minutes, such as in India and Nepal. Some areas in a time zone may use a different offset for part of the year, typically one hour ahead during spring and summer, a practice known as daylight saving time (DST).

Details

Time zone is a zone on the terrestrial globe that is approximately 15° longitude wide and extends from pole to pole and within which a uniform clock time is used. Time zones are the functional basis of standard time and were introduced in the late 19th century as railways connected places that had differing local times.

Different time zones occur because of the way in which Earth spins. Earth rotates 360 degrees every 24 hours, and therefore different parts of the planet experience daylight and darkness at different times. To coordinate time with daylight, the globe is divided into 24 segments, each 15 degrees of longitude apart. The prime meridian in Greenwich, England, serves as the starting point for these divisions, creating a global framework for timekeeping.

While the theoretical model of time zones is straightforward, practical adjustments are often made to accommodate political, social, and economic factors. For instance, some regions have chosen to adopt time offsets of 30 or 45 minutes instead of the standard one-hour difference. These adjustments are made to better align with local solar time or to unify time within a country, as seen in places such as Newfoundland, Iran, and India.

Before the concept of time zones, each locality set its time based on the Sun’s position, leading to a chaotic array of local times. This system became impractical with the advent of rapid railway transportation in the late 19th century, which required a more uniform timekeeping system to avoid confusion in scheduling. The introduction of standard time zones was a solution to this problem, allowing regions to adopt a consistent time standard.

International Date Line is an imaginary line extending between the North Pole and the South Pole and arbitrarily demarcating each calendar day from the next. It corresponds along most of its length to the 180th meridian of longitude but deviates eastward through the Bering Strait to avoid dividing Siberia and then deviates westward to include the Aleutian Islands with Alaska. South of the Equator, another eastward deviation allows certain island groups to have the same day as New Zealand.

The International Date Line is a consequence of the worldwide use of timekeeping systems arranged so that local noon corresponds approximately to the time at which the sun crosses the local meridian of longitude. A traveler going completely around the world while carrying a clock that he advanced or set back by one hour whenever he entered a new time zone and a calendar that he advanced by one day whenever his clock indicated midnight would find on returning to his starting point that the date according to his own experience was different by one day from that kept by persons who had remained at the starting point. The International Date Line provides a standard means of making the needed readjustment: travelers moving eastward across the line set their calendars back one day, and those traveling westward set theirs a day ahead.

Coordinated Universal Time (UTC) is the international basis of civil and scientific time, which was introduced on January 1, 1960. The unit of UTC is the atomic second, and UTC is widely broadcast by radio signals. These signals ultimately furnish the basis for the setting of all public and private clocks. Since January 1, 1972, UTC has been modified by adding “leap seconds” when necessary.

UTC serves to accommodate the timekeeping differences that arise between atomic time (which is derived from atomic clocks) and solar time (which is derived from astronomical measurements of Earth’s rotation on its axis relative to the Sun). UTC is thus kept within an exact number of seconds of International Atomic Time and is also kept within 0.9 second of the solar time denoted UT1. Because of the irregular slowing of Earth’s rate of rotation by tidal friction and other forces, there is now about one more (atomic clock-derived) second in a solar year than there are UT1 seconds. To remedy this discrepancy, UTC is kept within 0.9 second of UT1 by adding a leap second to UTC as needed; the last minute of December or June is made to contain 61 seconds. The slowing of Earth’s rotation varies irregularly, and so the number of leap seconds by which UTC must be retarded to keep it in epoch with UT1 cannot be predicted years in advance. Impending leap seconds for UTC are announced at least eight weeks in advance by the International Earth Rotation and Reference Systems Service at the Paris Observatory, however.

Universal Time (UT), the mean solar time of the Greenwich meridian (0° longitude). Universal Time replaced the designation Greenwich Mean Time in 1928; it is now used to denote the solar time (q.v. : quod vide : which see) when an accuracy of about one second suffices. In 1955 the International Astronomical Union defined several categories of Universal Time of successively increasing accuracy. UT0 represents the initial values of Universal Time obtained by optical observations of star transits at various astronomical observatories. These values differ slightly from each other because of the effects of polar motion (q.v.). UT1, which gives the precise angular coordinate of the Earth about its spin axis, is obtained by correcting UT0 for the effects of polar motion. Finally, an empirical correction to take account of annual changes in the Earth’s speed of rotation is added to UT1 to convert it into UT2. Coordinated Universal Time (q.v.), the international basis of civil and scientific time, is obtained from an atomic clock that is adjusted in epoch so as to remain close to UT1; in this way, the solar time that is indicated by Universal Time is kept in close coordination with atomic time.

Standard Time, the time of a region or country that is established by law or general usage as civil time.

The concept was adopted in the late 19th century in an attempt to end the confusion that was caused by each community’s use of its own solar time. Some such standard became increasingly necessary with the development of rapid railway transportation and the consequent confusion of schedules that used scores of different local times kept in separate communities. (Local time varies continuously with change in longitude.) The need for a standard time was felt most particularly in the United States and Canada, where long-distance railway routes passed through places that differed by several hours in local time. Sir Sandford Fleming, a Canadian railway planner and engineer, outlined a plan for worldwide standard time in the late 1870s. Following this initiative, in 1884 delegates from 27 countries met in Washington, D.C., and agreed on a system basically the same as that now in use.

The present system employs 24 standard meridians of longitude (lines running from the North Pole to the South Pole, at right angles to the Equator) 15° apart, starting with the prime meridian through Greenwich, England. These meridians are theoretically the centres of 24 Standard Time zones, although in practice the zones often are subdivided or altered in shape for the convenience of inhabitants; a notable example of such alteration is the eastward extension of the International Date Line around the Pacific island country of Kiribati. Time is the same throughout each zone and differs from the international basis of legal and scientific time, Coordinated Universal Time, by an integral number of hours; minutes and seconds are the same. In a few regions, however, the legal time kept is not that of one of the 24 Standard Time zones, because half-hour or quarter-hour differences are in effect there. In addition, Daylight Saving Time is a common system by which time is advanced one hour from Standard Time, typically to extend daylight hours during conventional waking time and in most cases for part of the year (usually in summer).

Additional Information

Data spotlights represent data and statistics from a specific period of time, and do not reflect ongoing data collection. As individual spotlights are static stories, they are not subject to the Bureau of Transportation Statistics (BTS) web standards and may not be updated after their publication date. Please contact BTS to request updated information.

Before the establishment of time zones in 1883, there were more than 144 local times in North America. The resulting time differences between adjacent towns and cities were not critical when it took days to travel from place to place. With the proliferation of railroads, faster travel became possible across large geographies, and travelers could sometimes arrive at an earlier local time than they had departed. Due to this lack of time standardization, train scheduling proved difficult to coordinate, resulting in missed connections and collisions. As a result, the major railroad companies began to operate on a coordinated system of four time zones starting in 1883.

Because the development of standardized time was transportation-driven, the government coordination of time zones was handled by transportation agencies. In 1918, the federal organization in charge of railroad regulation — the Interstate Commerce Commission (ICC) — was given the power to address coordination concerns. That year, five time zones were officially adopted as the US entered World War I: the Eastern, Central, Mountain, Pacific, and Alaska zones, all of which are still in use today. However, the need for coordination among all transportation modes became increasingly important after World War II. When the Department of Transportation was created by Congress in 1966, it was assigned “the responsibility of regulating, fostering, and promoting widespread and uniform adoption and observance of standardized time” within each time zone.

Daylight Saving Time (DST) was enacted as a legal requirement by the Uniform Time Act of 1966. Motivated by transportation improvements, this act mandated standard time within the existing time zones and established a permanent system of uniform DST, including the dates and times for twice yearly transitions. While State governments cannot independently change time zones or the length of DST, they can exempt themselves from DST, independent of DOT authority or permission. Nonetheless, DST is observed uniformly across the nation except in American Samoa, Guam, Northern Mariana Islands, Puerto Rico, the Virgin Islands, Hawaii, and most of Arizona.

Today, the Department of Transportation continues to oversee standard time due to its historical and contemporary importance in transportation and associated commercial activity. Time zone boundaries, established by law, can only be changed by the Secretary of Transportation upon a determination that the proposed adjustment serves the “convenience of commerce.” Per DOT policy, a petition requesting such a change must come from the highest political authorities in a State or locality. Several communities have requested changes to their time zone designation over the past two decades, the most recent being Mercer County, North Dakota in 2010, which chose to switch from Mountain to Central Time. Authorizing these changes and keeping track of the legally designated time zone for each area of the U.S. are key facets of the DOT’s oversight of uniform time observance, time zones, and DST.

In 2019, the Bureau of Transportation Statistics (BTS), in coordination with the Office of the General Counsel, created a digital geographic representation of the official written time zone delineations defined in the Code of Federal Regulations (CFR), Title 49, Subtitle A, Part 71 - Standard Time Zone Boundaries. Currently the United States and its territories have 9 time zone boundaries: Atlantic, Eastern, Central, Mountain, Pacific, Alaska, Hawaii–Aleutian, Samoa, and Chamorro.

The DOT Time Zone Boundary Geospatial layer is the verified digital representation of the current time zone delineations as written in the CFR, and is part of the National Transportation Atlas Database (NTAD). This layer provides the American public with detailed, reliable, and authoritative information on time-related authorities and time zone boundaries.

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