In the US time is divided into nine standard time zones covering the states and its possessions, with most of the United States observing daylight saving time for part of the year. The time zone boundaries and DST observance are under the authority of the Department of Transportation. Official and highly precise time keeping services are provided by two federal time agencies: a Department of Commerce agency, the National Institute of Standards and Technology (NIST), and its military counterpart, the United States Naval Observatory (USNO). The clocks run by these services are kept highly synchronized with each other as well as with those of international time keeping organizations.

US time zones are:
Hawaii-Aleutian Standard Time (HST)
Alaskan Standard Time (AKST)
Pacific Standard Time (PST)
Mountain Standard Time (MST)
Central Standard Time (CST)
Eastern Standard Time (EST)
Atlantic Standard Time (AST)
Samoa standard time (SST)
Chamorro standard time (ChST)

US time zone map

See the current time in various US Time Zones

Standard US Time Zones are currently defined at the federal level by law 15 U.S.C. §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. As of August 9, 2007, the standard time zones are defined in terms of hourly offsets from UTC.[1] Prior to this they were based upon the mean solar time at several meridians 15° apart west of Greenwich (GMT).