A time zone is an area which observes a uniform standard time for legal, commercial and social purposes. Time zones tend to follow the boundaries between countries and their subdivisions instead of strictly following longitude, because it is convenient for areas in frequent communication to keep the same time.
The history of standard time in the United States began November 18, 1883, when United States and Canadian railroads instituted standard time in time zones. Before then, time of day was a local matter, and most cities and towns used some form of local solar time, maintained by some well-known clock. The new standard time system was not immediately embraced by all.
The Eastern Time Zone (ET) is a time zone encompassing part or all of 23 states in the eastern part of the United States, parts of eastern Canada, the state of Quintana Roo in Mexico, Panama, Colombia, mainland Ecuador, Peru, and a small portion of westernmost Brazil in South America, along with certain Caribbean and Atlantic islands.
The Alaska Time Zone observes standard time by subtracting nine hours from Coordinated Universal Time (UTC−09:00). During daylight saving time its time offset is eight hours (UTC−08:00). The clock time in this zone is based on mean solar time at the 135th meridian west of the Greenwich Observatory.
Time in the United States, by law, is divided into nine standard time zones covering the states, territories and other 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, but no single map of those existed until the agency announced intentions to make one in September 2022. Official and highly precise timekeeping services (clocks) are provided by two federal agencies: the National Institute of Standards and Technology (NIST) ; and the United States Naval Observatory (USNO). The clocks run by these services are kept synchronized with each other as well as with those of other international timekeeping organizations.
Standard time is the synchronisation of clocks within a geographical region to a single time standard, rather than a local mean time standard. Generally, standard time agrees with the local mean time at some meridian that passes through the region, often near the centre of the region. Historically, standard time was established during the 19th century to aid weather forecasting and train travel. Applied globally in the 20th century, the geographical regions became time zones. The standard time in each time zone has come to be defined as an offset from Universal Time. A further offset is applied for part of the year in regions with daylight saving time.
The Atlantic Time Zone is a geographical region that keeps standard time—called Atlantic Standard Time (AST)—by subtracting four hours from Coordinated Universal Time (UTC), resulting in UTC−04:00. AST is observed in parts of North America and some Caribbean islands. During part of the year, some portions of the zone observe daylight saving time, referred to as Atlantic Daylight Time (ADT), by moving their clocks forward one hour to result in UTC−03:00. The clock time in this zone is based on the mean solar time of the 60th meridian west of the Greenwich Observatory.
South African Standard Time (SAST) is the time zone used by all of South Africa as well as Eswatini and Lesotho. The zone is two hours ahead of UTC (UTC+02:00) and is the same as Central Africa Time. Daylight saving time is not observed in either time zone. Solar noon in this time zone occurs at 30° E in SAST, effectively making Pietermaritzburg at the correct solar noon point, with Johannesburg and Pretoria slightly west at 28° E and Durban slightly east at 31° E. Thus, most of South Africa's population experience true solar noon at approximately 12:00 daily.
The time in China follows a single standard time offset of UTC+08:00, even though China spans almost five geographical time zones. The official national standard time is called Beijing Time domestically and China Standard Time (CST) internationally. Daylight saving time has not been observed since 1991.
Time in Canada, by regional law, is divided into six standard time zones covering the country's provinces and territories. Most regions operate on standard time from the first Sunday in November to the second Sunday in March and daylight saving time the rest of the year.
The U.S. state of Indiana is divided between Eastern and Central time zones. The official dividing line has generally moved progressively west from its original location on the Indiana–Ohio border, to a position dividing Indiana down the middle, and finally to its current location along much of the Indiana–Illinois border. In April 2006, several southwestern and northwestern counties reverted to Central time, although by late 2007 all but two had returned to Eastern time.
The majority of the United States observes daylight saving time, the practice of setting the clock forward by one hour when there is longer daylight during the day, so that evenings have more daylight and mornings have less. Exceptions include Arizona, Hawaii, and the overseas territories of American Samoa, Guam, the Northern Mariana Islands, Puerto Rico, and the United States Virgin Islands. The Uniform Time Act of 1966 established the system of uniform daylight saving time throughout the US.
Time in Arizona, as in all U.S. states, is regulated by the United States Department of Transportation as well as by state and tribal law.
Time in Massachusetts, as in all US states, is regulated by the United States Department of Transportation. Massachusetts is in the Eastern Time Zone (ET) and observes daylight saving time (DST).
Time in New Hampshire, as in all US states, is regulated by the United States Department of Transportation. New Hampshire is in the Eastern Time Zone (ET) and observes daylight saving time (DST).
Time in Rhode Island, as in all US states, is regulated by the United States Department of Transportation. Rhode Island is in the Eastern Time Zone (ET) and observes daylight saving time (DST).
The state of New York uses the Eastern Time Zone (UTC-05:00) with daylight saving time (UTC-04:00). Because the largest city in the state, New York City, is the largest city in the United States, and because the Eastern Time Zone is used in the nation's capital, Washington, D.C., in addition to the fact that a plurality of the nation's population is in that time zone, Eastern Time is the most commonly used time zone for television scheduling. Therefore, if a program airs at 7:00 PM Eastern Time, the network will usually advertise it by saying "Tonight at 7, 6 Central", or just "Tonight at 7".
Time in Connecticut, as in all US states, is regulated by the United States Department of Transportation. Connecticut is in the Eastern Time Zone (ET) and observes daylight saving time (DST).
Time in Vermont, as in all US states, is regulated by the United States Department of Transportation. Vermont is in the Eastern Time Zone (ET) and observes daylight saving time (DST).
Time in the Kingdom of the Netherlands is denoted by Central European Time (CET) during the winter as standard time in the Netherlands, which is one hour ahead of coordinated universal time (UTC+01:00), and Central European Summer Time (CEST) during the summer as daylight saving time, which is two hours ahead of coordinated universal time (UTC+02:00). The Caribbean Netherlands – which consist of the islands of Bonaire, Sint Eustatius and Saba – all observe Atlantic Standard Time (AST) year-round, which is four hours behind coordinated universal time (UTC−04:00).
