Oracle 时区问题

oracle中的tz_offset是用来返回给定时区与标准时区(UTC universal of time Coordinate 通用时间坐标)的偏移量，其参数可以是：
1.一个合法的时区名
2.一个离UTC的标准偏移量
3.关键字SessionTimeZone或者是DB_timezone
 
其中合法的时区名可以从系统视图V$TimeZone_names中的列TZname中的得到：
TZNAME
——————————————————————————————————————————–
Africa/Cairo
Africa/Tripoli
America/Adak
America/Anchorage
America/Atka
America/Chicago
America/Denver
America/Detroit
America/Edmonton
America/Ensenada
America/Fort_Wayne
America/Halifax
America/Havana
America/Indiana/Indianapolis
America/Indianapolis
America/Jamaica
America/Los_Angeles
America/Manaus
America/Mazatlan
America/Mexico_City
America/Montreal
America/New_York
America/Noronha
America/Phoenix
America/Porto_Acre
America/Regina
America/Santiago
America/Sao_Paulo
America/Shiprock
America/St_Johns
America/Tijuana
America/Vancouver
America/Whitehorse
America/Winnipeg
Asia/Calcutta
Asia/Hong_Kong
Asia/Istanbul
Asia/Jerusalem
Asia/Riyadh
Asia/Seoul
Asia/Shanghai
Asia/Singapore
Asia/Taipei
Asia/Tehran
Asia/Tel_Aviv
Asia/Tokyo
Atlantic/Reykjavik
Australia/ACT
Australia/Adelaide
Australia/Brisbane
Australia/Broken_Hill
Australia/Canberra
Australia/Darwin
Australia/Hobart
Australia/LHI
Australia/Lindeman
Australia/Lord_Howe
Australia/Melbourne
Australia/NSW
Australia/North
Australia/Perth
Australia/Queensland
Australia/South
Australia/Sydney
Australia/Tasmania
Australia/Victoria
Australia/West
Australia/Yancowinna
Brazil/Acre
Brazil/DeNoronha
Brazil/East
Brazil/West
CET
CST
CST6CDT
Canada/Atlantic
Canada/Central
Canada/East-Saskatchewan
Canada/Eastern
Canada/Mountain
Canada/Newfoundland
Canada/Pacific