SQL & PL/SQL

I know similar question has been asked many times in several forums. Most of them dealt with fetching interval or difference between timestamps in numbers (hours, mins,...) or using PL/SQL or custom functions.

Without using PL or custom functions, but just a sql what's the most efficient way of calculating timestamp that's half way between 2 timestamps?

alter session set nls_date_format = 'YYYY-MM-DD HH24:MI:SS';

select
to_date('2013-12-05 04:30:20','YYYY-MM-DD HH24:MI:SS') start_date,
to_date('2013-12-06 15:00:30','YYYY-MM-DD HH24:MI:SS') end_date,
'' avg_date
from dual;

Thanks,

-srinivas y.