<P> All U.S. insular territories with civilian government (American Samoa, Guam, Northern Mariana Islands, Puerto Rico, and the Virgin Islands) lie in the tropics and do not observe DST . </P> <P> While neighboring Samoa began observing DST in September 2010, the smaller American Samoa cannot legally follow because of the DST observation period mandated by the Uniform Time Act . This period is actually wintertime in this Southern Hemisphere territory . </P> <P> Many computer operating systems (such as Microsoft Windows, Linux, UNIX, and macOS) and programming languages (such as Java, perl, and most shell languages) allow a local time zone setting in the format of (standard time zone abbreviation) (UTC hour difference) (daylight saving time zone abbreviation). This allows programs and programming languages that must do calculations based on local time to more easily calculate differences between local time and UTC, as well as knowing whether calculations should be changed during daylight saving time . For example, a time zone setting of EST5EDT indicates that local time on the computer is 5 hours behind UTC and should be adjusted for daylight saving time . </P> <Table> <Tr> <Th> Time Zone </Th> <Th> Standard Time </Th> <Th> Daylight Saving Time </Th> <Th> TZ </Th> </Tr> <Tr> <Td> Atlantic Time Zone </Td> <Td> AST (UTC - 4) </Td> <Td> ADT (UTC - 3) </Td> <Td> AST4ADT </Td> </Tr> <Tr> <Td> Eastern Time Zone </Td> <Td> EST (UTC - 5) </Td> <Td> EDT (UTC - 4) </Td> <Td> EST5EDT </Td> </Tr> <Tr> <Td> Central Time Zone </Td> <Td> CST (UTC - 6) </Td> <Td> CDT (UTC - 5) </Td> <Td> CST6CDT </Td> </Tr> <Tr> <Td> Mountain Time Zone </Td> <Td> MST (UTC - 7) </Td> <Td> MDT (UTC - 6) </Td> <Td> MST7MDT </Td> </Tr> <Tr> <Td> Pacific Time Zone </Td> <Td> PST (UTC - 8) </Td> <Td> PDT (UTC - 7) </Td> <Td> PST8PDT </Td> </Tr> <Tr> <Td> Alaska Time Zone </Td> <Td> AKST (UTC - 9) </Td> <Td> AKDT (UTC - 8) </Td> <Td> AKST9AKDT </Td> </Tr> <Tr> <Td> Hawaii--Aleutian Time Zone </Td> <Td> HAST (UTC - 10) </Td> <Td> HADT (UTC - 9) Aleutian Islands only </Td> <Td> HAST10HADT </Td> </Tr> </Table>

Us cities that don't observe daylight savings time