Noon occurred at different times but time differences between distant locations were barely noticeable prior to the 19th century because of long travel times and the lack of long-distance instant communications prior to the development of the telegraph. See also: Standard time in the United Statesīefore the adoption of four standard 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 local mean solar time. History Rand McNally Standard Map of the United States, 1921, showing boundaries different from today It is the combination of the time zone and daylight saving rules, along with the timekeeping services, which determines the legal civil time for any U.S. The clocks run by these services are kept synchronized with each other as well as with those of other international timekeeping organizations. Official and highly precise timekeeping services (clocks) are provided by two 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 time zone boundaries and DST observance are regulated by the Department of Transportation, but no single map of those existed until the agency announced intentions to make one in September 2022.

In the United States, time is divided into nine standard time zones covering the states, territories and other US possessions, with most of the country observing daylight saving time (DST) for approximately the spring, summer, and fall months.

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