Please note that, this query makes use of a v$ table, thus keep in mind you must be logged in as a DBA.
select instance_name,to_char(startup_time,'mm/dd/yyyy hh24:mi:ss') as startup_time from v$instance;
Your output should look something like that following:
INSTANCE_NAME STARTUP_TIME ---------------- ------------------- orcl01 08/03/2008 00:23:08