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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.
Standard time is the synchronization 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 ...
One décime is equal to 10 decimal minutes, which is nearly equal to a quarter-hour (15 minutes) in standard time. Thus, "five hours two décimes" equals 5.2 decimal hours, roughly 12:30 p.m. in standard time. [8] [9] One hundredth of a decimal second was a decimal tierce. [10]
It is ultimately the authority of the secretary of transportation, in coordination with the states, to determine which regions will observe which of the standard time zones and if they will observe daylight saving time. [7] As of August 9, 2007, the standard time zones are defined in terms of hourly offsets from UTC. [8]
The Standard Time Act of 1918 was the first law to implement standard and daylight saving times at the federal level.
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.
Two U.S. standards mandate the use of year-month-day formats: ANSI INCITS 30-1997 (R2008); and NIST FIPS PUB 4-2 (FIPS PUB 4-2 withdrawn in United States 2008-09-02 [10] [11]), the earliest of which is traceable back to 1968. This is only required when compliance with the given standard is, or was, required.
UTC+10 highlighted on a world map. The Chamorro Time Zone, [1] formerly the Guam Time Zone, is a United States time zone which observes standard time ten hours ahead of Coordinated Universal Time . The clock time in this zone is based on the mean solar time of the 150th meridian east of the Greenwich Observatory.