Oracle 11g Release 2

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select (timestamp '1970-01-01 00:00:00.0 UTC' + (1539961967567/1000 * interval '1' second))
  at time zone 'America/Denver' as denver_time
from dual

DENVER_TIME

19-OCT-18 09.12.47.567000000 AM AMERICA/DENVER

select timestamp '2018-10-19 09:12:47.567 AMERICA/DENVER'
  - timestamp '1970-01-01 00:00:00.0 UTC' as diff
from dual;

DIFF

+000017823 15:12:47.567000000

select extract(day from diff) * 86400000
  + extract(hour from diff) * 3600000
  + extract(minute from diff) * 60000
  + extract(second from diff) * 1000 as unixtime
from (
  select timestamp '2018-10-19 09:12:47.567 AMERICA/DENVER'
    - timestamp '1970-01-01 00:00:00.0 UTC' as diff
  from dual
);

UNIXTIME

1539961967567