ABC / U.S. Time Zones

Standard time in the U.S. and its territories is observed within nine time zones. Standard time within each time zone is an integral number of hours offset from a time scale called Universal Time, Coordinated (abbreviated UTC), maintained by a large number of very precise "atomic clocks" at laboratories around the world, including the U.S. Naval Observatory.
To obtain U.S. civil time from UTC, use the following table. Time Zone Conversion
To obtain Atlantic Daylight Time subtract 3 hours from UTC
Atlantic Standard Time subtract 4 hours from UTC
Eastern Daylight Time subtract 4 hours from UTC
Eastern Standard Time subtract 5 hours from UTC
Central Daylight Time subtract 5 hours from UTC
Central Standard Time subtract 6 hours from UTC
Mountain Daylight Time subtract 6 hours from UTC
Mountain Standard Time subtract 7 hours from UTC
Pacific Daylight Time subtract 7 hours from UTC
Pacific Standard Time subtract 8 hours from UTC
Alaska Daylight Time subtract 8 hours from UTC
Alaska Standard Time subtract 9 hours from UTC
Hawaii-Aleutian Daylight Time subtract 9 hours from UTC
Hawaii-Aleutian Standard Time subtract 10 hours from UTC
Samoa Standard Time subtract 11 hours from UTC
Chamorro Standard Time add 10 hours to UTC

Thus, 11:00 UTC is the same as 5:00 CST. When converting zone time to or from UTC, dates must be properly taken into account. For example, 10 March at 02:00 UTC is the same as 9 March at 21:00 EST. The table can also be used to determine the difference between the time observed in any two zones. For example, the table shows that Eastern Standard Time is three hours "ahead" of Pacific Standard Time.

Time zones in the U.S. are defined in the U.S. Code, Title 15, Chapter 6, Subchapter IX - Standard Time. The Department of Transportation is responsible for time zone boundaries.

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