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Time in the United States, by law, is divided into nine normal time zones covering the states, territories and other US possessions, with a lot 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 companies: the National Institute of Standards and Technology (NIST) (an company 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 these of different worldwide timekeeping organizations.

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

Earlier than the adoption of four 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 remark, to form native mean solar time. Midday occurred at different times but time differences between distant locations had been barely discoverable prior to the 19th century because of long travel times and the lack of lengthy-distance prompt communications previous to the development of the telegraph.

Using local solar time grew to become more and more awkward as railways and telecommunications improved. American railroads maintained many different time zones through the late 1800s. Every train station set its own clock making it difficult to coordinate train schedules and complicated passengers. Time calculation became a critical problem for individuals touring by train (sometimes hundreds of miles in a day), based on the Library of Congress. Train drivers must recalculate their own clocks so as to know departure time. Every city in the United States used a special time commonplace so there were more than 300 local sun times to decide on from. Time zones were subsequently a compromise, stress-free the complicated geographic dependence while still permitting native 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 standard time zones for his weather stations, an concept which he offered to the railroads. Operators of the new railroad lines needed a new time plan that would provide a uniform train schedule for departures and arrivals. 4 normal time zones for the continental United States were introduced at midday 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 idea of Coordinated Universal Time (UTC), which turned the new worldwide civil time standard. UTC is, within about 1 second, mean solar time at zero°.[5] UTC does not observe daylight saving time.

For many functions, UTC is considered interchangeable with GMT, however GMT is no longer exactly defined by the scientific community. UTC is certainly one of several intently associated successors to GMT.

Standard time zones within the United States and other areas are at present defined at the federal level by law 15 USC §260. The federal law additionally establishes the transition dates and instances at which daylight saving time happens, if observed. It’s finally the creatority of the secretary of transportation, in coordination with the states, to find out which areas will observe which of the standard time zones and in the event that they will observe daylight saving time. As of August 9, 2007, the standard time zones are defined in terms of hourly offsets from UTC. Previous to this they have been based mostly upon the imply solar time at several meridians 15° apart west of Greenwich (GMT).

Only the total-time zone names listed beneath are official; abbreviations are by widespread 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 certainly one of guidelines over its entire territory:

Most use the standard time for their zone (or zones, the place a state is divided between two zones), aside from utilizing daylight saving time in the course of the summer season months. Initially this ran from the last Sunday in April until the last Sunday in October. Two subsequent amendments, in 1986 and 2005, have shifted today in order that daylight saving time now runs from the second Sunday in March until the first Sunday in November.

Arizona time zones

Arizona and Hawaii use normal time all through the year. Nevertheless:

The Navajo Nation observes DST all through its complete territory, including the portion that lies in Arizona. However the Hopi Nation, which is entirely surrounded by the Navajo Nation and is fully in Arizona, doesn’t observe DST.

In 2005, Indiana passed legislation that took impact on April 2, 2006, that placed all the state on daylight saving time (see Time in Indiana). Earlier than then, Indiana officially used standard time yr-round, with the following exceptions:

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

Some Indiana counties near Cincinnati and Louisville had been on japanese time (ET) but did (unofficially) observe DST.

The data from Indiana switching to DST shows DST does not really save any energy and in contrast really leads to elevated energy use

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

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