Since you did not say what version of Oracle you are using I'm going to assume 9i.

I have not tested this, but it should get you on the right track.

declare
epoch timestamp with time zone;
other_date timestamp with time zone;
interval number;
begin
interval := 1000000000;
epoch := timestamp '1970-01-01 00:00:00.000000 GMT';
other_date := epoch + interval / 3600 / 24;
dbms_output.put_line(to_char(other_date));
end;
/

--
Paul