According to metalink, you need to run this for 7.3

select to_char(to_date(value, 'J'), 'DD-MM-YYYY HH24:MM:SS')
from v$instance,
where key = 'STARTUP TIME - JULIAN'
/

but this gave me the wrong value, so I tried linking the table to itself - since I guess you then need seconds past midnight:

select to_char(to_date(a.value, 'J') + to_date(b.value, 'SSSS'), 'DD-MM-YYYY HH24:MM:SS')
from v$instance a, v$instance b
where a.key = 'STARTUP TIME - JULIAN'
and b.key = 'STARTUP TIME - SECONDS'

but this said I can't add date + date (I'm not to good with date funtions!).

My alert.log shows startup at 22:22 on 16th Nov.