oracle convert unix epoch time to date

Another option is to use an interval type:

SELECT TO_TIMESTAMP('1970-01-01 00:00:00.0'
                   ,'YYYY-MM-DD HH24:MI:SS.FF'
       ) + NUMTODSINTERVAL(1493963084212/1000, 'SECOND')
FROM dual;

It has this advantage that milliseconds won't be cut.

Here it is for both UTC/GMT and EST;

GMT  select (to_date('1970-01-01 00','yyyy-mm-dd hh24') +
     (1519232926891)/1000/60/60/24) from dual;

EST  select new_time(to_date('1970-01-01 00','yyyy-mm-dd hh24') + 
     (1519232926891)/1000/60/60/24, 'GMT', 'EST') from dual;

To convert from milliseconds from epoch (assume epoch is Jan 1st 1970):

select to_date('19700101', 'YYYYMMDD') + ( 1 / 24 / 60 / 60 / 1000) * 1322629200000
from dual;

11/30/2011 5:00:00 AM

To convert that date back to milliseconds:

select (to_date('11/30/2011 05:00:00', 'MM/DD/YYYY HH24:MI:SS') - to_date('19700101', 'YYYYMMDD')) * 24 * 60 * 60 * 1000
from dual;

1322629200000

If its seconds instead of milliseconds, just omit the 1000 part of the equation:

select to_date('19700101', 'YYYYMMDD') + ( 1 / 24 / 60 / 60 ) * 1322629200
from dual;

select (to_date('11/30/2011 05:00:00', 'MM/DD/YYYY HH24:MI:SS') - to_date('19700101', 'YYYYMMDD')) * 24 * 60 * 60
from dual;

Hope that helps.