The following query works well with Oracle 9i. i’ve no clue whether this works with others too or not.

SELECT TO_CHAR (startup_time, 'DD-MON-YYYY HH24:MI:SS') "DB Startup Time"
  FROM SYS.v_$instance;

even there is possiblity to find the startup time by quering logon_time from sys.v_$session view.