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 adoption of standard time, because of the inseparable correspondence between time and longitude, solidified the concept of halving the globe into an eastern and western hemisphere, with one prime meridian replacing the various prime meridians that had previously been used.
During the 19th century, scheduled steamships and trains required time standardisation in the industrialized world.
A standardised time system was first used by British railways on 1 December 1847, when they switched from local mean time, which varied from place to place, to Greenwich Mean Time (GMT). It was also given the name railway time, reflecting the important role the railway companies played in bringing it about. The vast majority of Great Britain's public clocks were standardised to GMT by 1855.
Until 1883, each United States railroad chose its own time standards. The Pennsylvania Railroad used the "Allegheny Time" system, an astronomical timekeeping service which had been developed by Samuel Pierpont Langley at the University of Pittsburgh’s Allegheny Observatory (then known as the Western University of Pennsylvania, located in Pittsburgh, Pennsylvania). Instituted in 1869, the Allegheny Observatory's service is believed to have been the first regular and systematic system of time distribution to railroads and cities as well as the origin of the modern standard time system.  By 1870 the Allegheny Time service extended over 2,500 miles with 300 telegraph offices receiving time signals. 
However, almost all railroads out of New York ran on New York time, and railroads west from Chicago mostly used Chicago time, but between Chicago and Pittsburgh/Buffalo the norm was Columbus time, even on railroads such as the PFtW&C and LS&MS, which did not run through Columbus. The Santa Fe Railroad used Jefferson City (Missouri) time all the way to its west end at Deming, New Mexico, as did the east–west lines across Texas; Central Pacific and Southern Pacific Railroads used San Francisco time all the way to El Paso. The Northern Pacific Railroad had seven time zones between St. Paul and the 1883 west end of the railroad at Wallula Jct; the Union Pacific Railway was at the other extreme, with only two time zones between Omaha and Ogden. 
In 1870, Charles F. Dowd proposed four time zones based on the meridian through Washington, DC for North American railroads.  In 1872 he revised his proposal to base it on the Greenwich meridian. Sandford Fleming, a Scottish-born Canadian engineer, proposed worldwide Standard Time at a meeting of the Royal Canadian Institute on February 8, 1879.  Cleveland Abbe advocated standard time to better coordinate international weather observations and resultant weather forecasts, which had been coordinated using local solar time. In 1879 he recommended four time zones across the contiguous United States, based upon Greenwich Mean Time.  The General Time Convention (renamed the American Railway Association in 1891), an organization of US railroads charged with coordinating schedules and operating standards, became increasingly concerned that if the US government adopted a standard time scheme it would be disadvantageous to its member railroads. William F. Allen, the Convention secretary, argued that North American railroads should adopt a five-zone standard, similar to the one in use today, to avoid government action. On October 11, 1883, the heads of the major railroads met in Chicago at the Grand Pacific Hotel  and agreed to adopt Allen's proposed system.
The members agreed that on Sunday, November 18, 1883, all United States and Canadian railroads would readjust their clocks and watches to reflect the new five-zone system on a telegraph signal from the Allegheny Observatory in Pittsburgh at exactly noon on the 90th meridian.    Although most railroads adopted the new system as scheduled, some did so early on October 7 and others late on December 2. The Intercolonial Railway serving the Canadian maritime provinces of New Brunswick and Nova Scotia just east of Maine decided not to adopt Intercolonial Time based on the 60th meridian west of Greenwich, instead adopting Eastern Time, so only four time zones were actually adopted by U.S./Canadian railroads in 1883.  Major American observatories, including the Allegheny Observatory, the United States Naval Observatory, the Harvard College Observatory, and the Yale University Observatory, agreed to provide telegraphic time signals at noon Eastern Time.  
Standard time was not enacted into US law until the 1918 Standard Time Act established standard time in time zones; the law also instituted daylight saving time (DST). The daylight saving time portion of the law was repealed in 1919 over a presidential veto, but was re-established nationally during World War II.   In 2007 the US enacted a federal law formalising the use of Coordinated Universal Time as the basis of standard time, and the role of the Secretary of Commerce (effectively, the National Institute of Standards and Technology) and the Secretary of the Navy (effectively, the U.S. Naval Observatory) in interpreting standard time. 
In 1999, standard time was inducted into the North America Railway Hall of Fame in the category "National: Technical Innovations." 
The Dominion of Newfoundland, whose capital St. John's falls almost exactly midway between the meridians anchoring the Atlantic Time Zone and the Greenland Time Zone, voted[ when? ] to create a half-hour offset time zone known as the Newfoundland Time Zone, at three and a half hours behind Greenwich time.
In the Netherlands, introduction of the railways made it desirable to create a standard time. On 1 May 1909, Amsterdam Time or Dutch Time was introduced. Before that, time was measured in different cities; in the east of the country, this was a few minutes earlier than in the west. After that, all parts of the country had the same local time—that of the Wester Tower in Amsterdam (Westertoren/4°53'01.95" E). This time was indicated as GMT +0h 19m 32.13s until 17 March 1937, after which it was simplified to GMT+0h20m. This time zone was also known as the Loenen time or Gorinchem time, as this was the exact time in both Loenen and Gorinchem. At noon in Amsterdam, it was 11:40 in London and 12:40 in Berlin.
The shift to the current Central European Time zone took place on 16 May 1940. The German occupiers ordered the clock to be moved an hour and forty minutes forward. This time was kept in summer and winter throughout 1941 and 1942. It was only in November 1942 that a different Winter time was introduced, and the time was adjusted one hour backwards. This lasted for only three years; after the liberation of the Netherlands in 1945, Summer time was abolished for over thirty years, so during those years, standard time was 40 minutes ahead of the original Amsterdam Time. As of 2017, the Netherlands is in line with Central European Time (GMT+1 in the winter, GMT+2 in the summer, which is significantly different from Amsterdam Time).
In 1868, New Zealand was the first country in the world to establish a nationwide standard time. 
A telegraph cable between New Zealand's two main islands became the instigating factor for the establishment of "New Zealand time". In 1868, the Telegraph Department adopted "Wellington time" as the standard time across all their offices so that opening and closing times could be synchronised. The Post Office, which usually shared the same building, followed suit. However, protests that time was being dictated by one government department, led to a resolution in parliament to establish a standard time for the whole country.
The director of the Geological Survey, James Hector, selected New Zealand time to be at the meridian 172°30′E. This was very close to the country's mean longitude and exactly 11+1⁄2 hours in advance of Greenwich Mean Time. It came into effect on 2 November 1868.
For over fifty years, the Colonial Time Service Observatory in Wellington, determined the correct time each morning. At 9 a.m. each day, it was transmitted by Morse code to post offices and railway stations around the country. In 1920, radio time signals began broadcasting, greatly increasing the accuracy of the time nationwide.
Greenwich Mean Time (GMT) is the mean solar time at the Royal Observatory in Greenwich, London, counted from midnight. At different times in the past, it has been calculated in different ways, including being calculated from noon; as a consequence, it cannot be used to specify a particular time unless a context is given. The term 'GMT' is also used as one of the names for the time zone UTC+00:00 and, in UK law, is the basis for civil time in the United Kingdom.
A time zone is an area that 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.
Universal Time is a time standard based on Earth's rotation. While originally it was mean solar time at 0° longitude, precise measurements of the Sun are difficult. Therefore, UT1 is computed from a measure of the Earth's angle with respect to the International Celestial Reference Frame (ICRF), called the Earth Rotation Angle. UT1 is the same everywhere on Earth. UT1 is required to follow the relationship
The International Meridian Conference was a conference held in October 1884 in Washington, D.C., in the United States, to determine a prime meridian for international use. The conference was held at the request of U.S. President Chester A. Arthur. The subject to discuss was the choice of "a meridian to be employed as a common zero of longitude and standard of time reckoning throughout the world". It resulted in the recommendation of the Greenwich Meridian as the international standard for zero degrees longitude.
Japan Standard Time, or Japan Central Standard Time, is the standard time zone in Japan, 9 hours ahead of UTC. Japan does not observe daylight saving time, though its introduction has been debated on several occasions. During World War II, the time zone was often referred to as Tokyo Standard 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.
Indian Standard Time (IST), sometimes also called India Standard Time, is the time zone observed throughout India, with a time offset of UTC+05:30. India does not observe daylight saving time or other seasonal adjustments. In military and aviation time IST is designated E* ("Echo-Star"). It is indicated as Asia/Kolkata in the IANA time zone database.
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. 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.
Time in New Zealand is divided by law into two standard time zones. The main islands use New Zealand Standard Time (NZST), 12 hours in advance of Coordinated Universal Time (UTC) / military M (Mike), while the outlying Chatham Islands use Chatham Standard Time (CHAST), 12 hours 45 minutes in advance of UTC / military M^ (Mike-Three).
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.
Hong Kong Time is the time in Hong Kong, observed at UTC+08:00 all year round. The Hong Kong Observatory is the official timekeeper of the Hong Kong Time. It is indicated as Asia/Hong_Kong in the IANA time zone database.
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.
Charles F. Dowd (1825–1904) was a co-principal of the Temple Grove Ladies Seminary in Saratoga Springs, New York. He was the first person to propose multiple time zones for any country, those for the railways of the United States. He did not propose their extension to the entire world, which was suggested by the Italian mathematician Quirico Filopanti, and championed by the Canadian engineer Sandford Fleming.
Canada is divided into six time zones. 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.
Thailand follows UTC+07:00, which is 7 hours ahead of UTC. The local mean time in Bangkok was originally UTC+06:42:04. Thailand used this local mean time until 1920, when it changed to Indochina Time, UTC+07:00; ICT is used all year round as Thailand does not observe daylight saving time. Thailand shares the same time zone with Vietnam, Cambodia, Laos, Christmas Island, and Western Indonesia.
Malaysian Standard Time or Malaysian Time (MYT) is the standard time used in Malaysia. It is 8 hours ahead of Coordinated Universal Time (UTC). The local mean time in Kuala Lumpur was originally GMT+06:46:46. Peninsular Malaysia used this local mean time until 1 January 1901, when they changed to Singapore mean time GMT+06:55:25. Between the end of the Second World War and the formation of Malaysia on 16 September 1963, it was known as British Malayan Standard Time, which was GMT+07:30. At 2330 hrs local time of 31 December 1981, people in Peninsular Malaysia adjusted their clocks and watches ahead by 30 minutes to become 00:00 hours local time of 1 January 1982, to match the time in use in East Malaysia, which is UTC+08:00. SGT (Singapore) followed on and uses the same until now.
Railway time was the standardised time arrangement first applied by the Great Western Railway in England in November 1840, the first recorded occasion when different local mean times were synchronised and a single standard time applied. The key goals behind introducing railway time were to overcome the confusion caused by having non-uniform local times in each town and station stop along the expanding railway network and to reduce the incidence of accidents and near misses, which were becoming more frequent as the number of train journeys increased.
Denmark, including the dependencies Faroe Islands and Greenland, uses six time zones.
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).
Finland uses Eastern European Time (EET) during the winter as standard time and Eastern European Summer Time (EEST) during the summer as daylight saving time. EET is two hours ahead of coordinated universal time (UTC+02:00) and EEST is three hours ahead of coordinated universal time (UTC+03:00). Finland adopted EET on 30 April 1921, and has observed daylight saving time in its current alignment since 1981 by advancing the clock forward one hour at 03:00 EET on the last Sunday in March and back at 04:00 EET on the last Sunday in October, doing so an hour earlier for the first two years.