SYS_EXTRACT_UTC
SYS_EXTRACT_UTC(x) converts the TIMESTAMP WITH TIMEZONEx to a TIMESTAMP in UTC.
SQL> SELECT SYS_EXTRACT_UTC(TIMESTAMP '2012-12-17 21:14:21 PST') FROM dual;
SYS_EXTRACT_UTC(TIMESTAMP'2012-12-1721:14:21PST')
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18-DEC-12 05.14.21.000000000 AM
SQL>