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Time in the United States, by law, is split into nine commonplace time zones covering the states, territories and different US possessions, with most of the United States observing daylight saving time (DST) for approximately the spring, summer season, and fall months. The time zone boundaries and DST observance are regulated by the Department of Transportation. Official and highly precise timekeeping providers (clocks) are provided by federal agencies: the National Institute of Standards and Technology (NIST) (an agency of the Department of Commerce); and the United States Naval Observatory (USNO). The clocks run by these providers are kept synchronized with each other as well as with those of other international timekeeping organizations.

It is the combination of the time zone and daylight saving guidelines, alongside with the timekeeping services, which determines the legal civil time for any U.S. location at any moment.

Earlier than the adoption of 4 standard time zones for the continental United States, many towns and cities set their clocks to midday when the sun passed their native meridian, pre-corrected for the equation of time on the date of statement, to form native mean solar time. Midday occurred at totally different occasions however time variations between distant places have been barely discoverable prior to the 19th century because of lengthy journey instances and the lack of lengthy-distance on the spot communications prior to the development of the telegraph.

The use of native solar time grew to become increasingly awkward as railways and telecommunications improved. American railroads maintained many different time zones throughout the late 1800s. Every train station set its own clock making it troublesome to coordinate train schedules and complicated passengers. Time calculation turned a serious problem for people touring by train (typically hundreds of miles in a day), in line with the Library of Congress. Train drivers must recalculate their own clocks as a way to know departure time. Every city in the United States used a unique time standard so there were more than 300 local sun occasions to decide on from. Time zones have been therefore a compromise, enjoyable the complicated geographic dependence while still permitting native time to be approximate with mean solar time. Railroad managers tried to address the problem by establishing a hundred railroad time zones, but this was only a partial resolution to the problem.

Weather service chief Cleveland Abbe introduced 4 standard time zones for his climate stations, an concept which he offered to the railroads. Operators of the new railroad lines needed a new time plan that will supply a uniform train schedule for departures and arrivals. Four commonplace time zones for the continental United States have been launched at midday on November 18, 1883, in Chicago, IL, when the telegraph lines transmitted time signals to all main cities.

From GMT to UTC

In 1960, the Worldwide Radio Consultative Committee formalized the idea of Coordinated Common Time (UTC), which grew to become the new international civil time standard. UTC is, within about 1 second, imply solar time at 0°.[5] UTC doesn’t observe daylight saving time.

For most functions, UTC is considered interchangeable with GMT, but GMT is not precisely defined by the scientific community. UTC is one in all a number of intently related successors to GMT.

Normal time zones in the United States and other regions are at present defined at the federal level by law 15 USC §260. The federal law also establishes the transition dates and occasions at which daylight saving time happens, if observed. It’s in the end the writerity of the secretary of transportation, in coordination with the states, to find out which regions will observe which of the standard time zones and if they will observe daylight saving time. As of August 9, 2007, the standard time zones are defined when it comes to hourly offsets from UTC. Prior to this they had been primarily based upon the mean solar time at a number of meridians 15° apart west of Greenwich (GMT).

Only the full-time zone names listed beneath are official; abbreviations are by widespread use conventions, and duplicated elsewhere on this planet for different time zones.

Daylight saving time (DST) begins on the second Sunday of March and ends on the primary Sunday of November.

In response to the Uniform Time Act of 1966, each state has formally chosen to apply one in all two guidelines over its total territory:

Most use the usual time for their zone (or zones, where a state is divided between two zones), aside from using daylight saving time through the summer time months. Initially this ran from the final Sunday in April till the final Sunday in October. Two subsequent amendments, in 1986 and 2005, have shifted nowadays in order that daylight saving time now runs from the second Sunday in March till the primary Sunday in November.

Arizona time zones

Arizona and Hawaii use standard time throughout the year. However:

The Navajo Nation observes DST throughout its whole territory, together with the portion that lies in Arizona. But the Hopi Nation, which is completely surrounded by the Navajo Nation and is totally in Arizona, does not observe DST.

In 2005, Indiana passed laws that took impact on April 2, 2006, that positioned the whole state on daylight saving time (see Time in Indiana). Before then, Indiana officially used normal time 12 months-spherical, with the next exceptions:

The parts of Indiana that have been on central time noticed daylight saving time.

Some Indiana counties close to Cincinnati and Louisville had been on jap time (ET) but did (unofficially) observe DST.

The data from Indiana switching to DST shows DST doesn’t actually save any energy and in contrast actually ends in elevated energy use

The Energy Policy Act of 2005 prolonged daylight saving time (DST) for an additional month starting in 2007.

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