Our billing system date format is using unix epoch time.

converison like the following:

oracle time --> unix epoch time
select (sysdate - to_date('1970-01-01 00:00:00','YYYY-MM-DD HH24:MI:SS')) * 86400 - substr(TZ_OFFSET(sessiontimezone),1,instr(TZ_OFFSET(sessiontimezone),':')-1) * 3600 from dual;

unix epoch time-->oracle time
select from_tz(cast(to_date('1970-01-01 00:00:00','YYYY-MM-DD HH24:MI:SS') as timestamp),'Australia/NSW') at local + numtodsinterval(1305882706,'SECOND') from dual;

you can find your own timezone from here: select distinct tzname from V$TIMEZONE_NAMES;

The easiest way, just use this webpage to convert: http://www.4webhelp.net/us/timestamp.php?action=stamp&stamp=1305917878&timezone=10