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Time in the United States, by law, is divided into nine standard time zones covering the states and its possessions, with most of the United States observing daylight saving time 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) (an agency of the Department of Commerce); and its military counterpart, 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. 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. == United States time zones == Standard time zones in the United States are currently defined at the federal level by law 15 USC §260.〔(15 USC §260 ).〕 The federal law also establishes the transition dates and times at which daylight saving time occurs, if observed. 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.〔(15 USC 260–262 )〕 As of August 9, 2007, the standard time zones are defined in terms of hourly offsets from UTC.〔(H.R.2272 — America COMPETES Act ) (Sec. 3013)〕 Prior to this they were based upon the mean solar time at several meridians 15° apart west of Greenwich (GMT). Only the full-time zone names listed below are official; abbreviations are by common use conventions, and duplicated elsewhere in the world for different time zones. The United States uses nine standard time zones. As defined by US law〔(Standard Time Zone Boundaries 49CFR71 )〕 they are: # Atlantic Time Zone # Eastern Time Zone # Central Time Zone # Mountain Time Zone # Pacific Time Zone # Alaska Time Zone # Hawaii–Aleutian Time Zone # Samoa Time Zone (UTC−11) # Chamorro Time Zone (UTC+10) View the standard time zone boundaries (here ). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Time in the United States」の詳細全文を読む スポンサード リンク
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