how to check date of Oracle installation

Hi All,
I need your advice ,please.
I was asked to check the date of Oracle installation.

The situation is:
It is a solaris machine.
I have checked the db and the oracle folders.
The folders were created on 2005
and (i checked some system table in db from all_objects) the last db
was created one year later(2006).
The latest version i have seen in db was enterprise edition.
How can I check if it was upgraded from standart and the
most important whether the enterprise edition was installed on 2005 or 2006?


thanks in advance

check the install logs