Time within the United States, by law, is split into nine customary time zones covering the states, territories and other US possessions, with many of the United States observing daylight saving time (DST) for approximately the spring, summer time, and fall months. The time zone boundaries and DST observance are regulated by the Department of Transportation. Official and highly exact timekeeping companies (clocks) are provided by two federal companies: 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 companies are kept synchronized with each other as well as with those of different international timekeeping organizations.
It is the mixture of the time zone and daylight saving rules, alongside with the timekeeping services, which determines the authorized civil time for any U.S. location at any moment.
Earlier than the adoption of four customary time zones for the continental United States, many towns and cities set their clocks to noon when the sun passed their local meridian, pre-corrected for the equation of time on the date of observation, to form native mean solar time. Noon happenred at totally different instances however time differences between distant places were barely noticeable prior to the nineteenth century because of long travel instances and the lack of lengthy-distance instantaneous communications prior to the development of the telegraph.
The use of local solar time turned more and more awkward as railways and telecommunications improved. American railroads maintained many alternative time zones throughout the late 1800s. Every train station set its own clock making it tough to coordinate train schedules and confusing passengers. Time calculation turned a serious problem for people traveling by train (sometimes hundreds of miles in a day), in line with the Library of Congress. Train drivers must recalculate their own clocks to be able to know departure time. Each city in the United States used a different time commonplace so there have been more than 300 native sun times to choose from. Time zones were due to this fact a compromise, enjoyable the advanced geographic dependence while still allowing local time to be approximate with imply solar time. Railroad managers tried to address the problem by establishing a hundred railroad time zones, however this was only a partial resolution to the problem.
Weather service chief Cleveland Abbe introduced four customary time zones for his climate stations, an thought which he offered to the railroads. Operators of the new railroad lines wanted a new time plan that will provide a uniform train schedule for departures and arrivals. Four customary time zones for the continental United States have been launched at noon on November 18, 1883, in Chicago, IL, when the telegraph lines transmitted time signals to all major cities.
From GMT to UTC
In 1960, the International Radio Consultative Committee formalized the concept of Coordinated Common Time (UTC), which became the new worldwide civil time standard. UTC is, within about 1 second, imply solar time at 0°.[5] UTC doesn’t observe daylight saving time.
For many functions, UTC is considered interchangeable with GMT, but GMT is no longer exactly defined by the scientific community. UTC is one among several carefully associated successors to GMT.
Customary time zones within the United States and other regions are presently defined at the federal level by law 15 USC §260. The federal law also establishes the transition dates and instances at which daylight saving time happens, if observed. It is ultimately the writerity of the secretary of transportation, in coordination with the states, to determine which areas will observe which of the standard time zones and if they will observe daylight saving time. As of August 9, 2007, the usual time zones are defined in terms of hourly offsets from UTC. Previous to this they had been primarily based upon the imply solar time at a number of meridians 15° apart west of Greenwich (GMT).
Only the total-time zone names listed under are official; abbreviations are by frequent use conventions, and duplicated elsewhere in the world 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, every state has officially chosen to apply one in all two rules over its whole territory:
Most use the usual time for his or her zone (or zones, where a state is divided between two zones), except for utilizing daylight saving time throughout the summer 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 as of late 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 normal time all through the year. However:
The Navajo Nation observes DST throughout its total territory, together with the portion that lies in Arizona. However the Hopi Nation, which is entirely surrounded by the Navajo Nation and is totally in Arizona, doesn’t observe DST.
In 2005, Indiana passed legislation that took effect on April 2, 2006, that placed all the state on daylight saving time (see Time in Indiana). Before then, Indiana officially used customary time yr-round, with the following exceptions:
The parts of Indiana that have been on central time noticed daylight saving time.
Some Indiana counties close to Cincinnati and Louisville have been on jap time (ET) but did (unofficially) observe DST.
The data from Indiana switching to DST shows DST doesn’t really save any energy and in contrast truly leads to increased energy use
The Energy Policy Act of 2005 extended daylight saving time (DST) for an additional month beginning in 2007.
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