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The evolution of United States standard time zone boundaries from 1919 to 2024 in five-year increments. Plaque in Chicago marking the creation of the four time zones of the continental US in 1883 Colorized 1913 time zone map of the United States, showing boundaries very different from today Map of U.S. time zones during between April 2, 2006, and March 11, 2007.
Before 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. Noon occurred at different times but time differences between distant ...
[48] [49] They allow a program to fetch the system time as UTC, represented as a year, month, day, hour, minute, second, and millisecond; [50] [51] Windows 95 and later, and Windows NT 3.5 and later, also allow the system time to be fetched as a count of 100 ns units since 1601-01-01 00:00:00 UTC.
Arthur C. Clarke proposed the use of a single time zone in 1976. [2] Attempts to abolish time zones date back half a century [1] and include the Swatch Internet Time. Economics professor Steve Hanke and astrophysics professor Dick Henry at Johns Hopkins University have been proponents of the concept and have integrated it in their Hanke–Henry Permanent Calendar.
Clock on The Exchange, Bristol, showing two minute hands, one for London time and one for Bristol time (GMT minus 11 minutes).. 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 Standard Time Act of 1918, also known as the Calder Act, was the first United States federal law implementing Standard time and Daylight saving time in the United States. [2] It defined five time zones for the continental United States and authorized the Interstate Commerce Commission to define the limits of each time zone.
For example, the year cannot be divided into twelve 28-day months since 12 times 28 is 336, well short of 365. The lunar month (as defined by the moon's rotation) is not 28 days but 28.3 days. The year, defined in the Gregorian calendar as 365.2425 days has to be adjusted with leap days and leap seconds .
A calendar era is the period of time elapsed since one epoch of a calendar and, if it exists, before the next one. [1] For example, the current year is numbered 2025 in the Gregorian calendar, which numbers its years in the Western Christian era (the Coptic Orthodox and Ethiopian Orthodox churches have their own Christian eras).