<P> 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. location at any moment . </P> <P> Prior to the adoption of four standard time zones for the continental United States, many towns and cities set clocks based on sunsets and sunrises at their locations and by observing the Sun's zenith at noon (mean solar time). Dawn and dusk occur 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 . </P> <P> The use of local solar time became increasingly awkward as railways and telecommunications improved . American railroads maintained many different time zones during the late 1800s . Each train station set its own clock making it difficult to coordinate train schedules and confusing passengers . Time calculation became a serious problem for people travelling by train (sometimes hundreds of miles in a day), according to the Library of Congress . Every city in the United States used a different time standard so there were more than 300 local sun times to choose from . Time zones were therefore a compromise, relaxing the complex geographic dependence while still allowing local time to be approximate with mean solar time . Railroad managers tried to address the problem by establishing 100 railroad time zones, but this was only a partial solution to the problem . </P> <P> Operators of the new railroad lines needed a new time plan that would offer a uniform train schedule for departures and arrivals . Four standard time zones for the continental United States were introduced at noon on November 18, 1883, when the telegraph lines transmitted time signals to all major cities . </P>

Where does the east coast time zone start