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Time in the United States, by law, is divided into nine standard 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, 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 two federal businesses: 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 companies are kept synchronized with one another as well as with those of other worldwide timekeeping organizations.

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

Before the adoption of 4 normal time zones for the continental United States, many towns and cities set their clocks to midday when the sun passed their local meridian, pre-corrected for the equation of time on the date of remark, to form native imply solar time. Midday occurred at different instances but time differences between distant places had been barely discoverable prior to the 19th century because of lengthy travel times and the lack of long-distance instantaneous communications previous to the development of the telegraph.

The use of local solar time became increasingly awkward as railways and telecommunications improved. American railroads maintained many alternative time zones through the late 1800s. Every train station set its own clock making it difficult to coordinate train schedules and confusing passengers. Time calculation turned a critical problem for folks touring by train (typically hundreds of miles in a day), according to the Library of Congress. Train drivers must recalculate their own clocks with a view to know departure time. Each city within the United States used a unique time customary so there have been more than 300 native sun occasions to choose from. Time zones had been due to this fact a compromise, relaxing the complicated geographic dependence while still permitting local time to be approximate with imply 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.

Climate service chief Cleveland Abbe introduced four normal time zones for his weather stations, an thought which he offered to the railroads. Operators of the new railroad lines needed a new time plan that will offer a uniform train schedule for departures and arrivals. 4 commonplace time zones for the continental United States had 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 Worldwide Radio Consultative Committee formalized the concept of Coordinated Common Time (UTC), which turned the new international civil time standard. UTC is, within about 1 second, mean solar time at zero°.[5] UTC doesn’t observe daylight saving time.

For many functions, UTC is considered interchangeable with GMT, however GMT is not exactly defined by the scientific community. UTC is certainly one of a number of closely related successors to GMT.

Normal time zones within the United States and other areas are at the moment 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 authority of the secretary of transportation, in coordination with the states, to determine which regions 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 usual time zones are defined when it comes to hourly offsets from UTC. Prior to this they have been based mostly upon the mean solar time at a number of meridians 15° apart west of Greenwich (GMT).

Only the full-time zone names listed below are official; abbreviations are by common 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 among guidelines over its complete territory:

Most use the usual time for his or her zone (or zones, where a state is split between two zones), apart from utilizing daylight saving time in the course of the summer time months. Originally this ran from the last Sunday in April till the last Sunday in October. Two subsequent amendments, in 1986 and 2005, have shifted lately in order that daylight saving time now runs from the second Sunday in March until the primary Sunday in November.

Arizona time zones

Arizona and Hawaii use customary time all through the year. However:

The Navajo Nation observes DST throughout its whole territory, including the portion that lies in Arizona. But the Hopi Nation, which is totally surrounded by the Navajo Nation and is entirely in Arizona, doesn’t observe DST.

In 2005, Indiana passed legislation that took impact on April 2, 2006, that placed the complete state on daylight saving time (see Time in Indiana). Earlier than then, Indiana formally used commonplace time 12 months-round, with the following exceptions:

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

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

The data from Indiana switching to DST shows DST does not actually save any energy and in contrast truly results in increased energy use

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

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