Use this command:
ls -al /var/log/installer/syslog
Creation date of installer log can tell you the installation date (since it's not modified after that).
Found here: http://ubuntuforums.org/showpost.php?p=10591868&postcount=5
Another solution is using this command:
ls -lt /usr/share/doc
Above of the default documents you can find creation date of documents for first installed applications.
I'm using ubuntu 10.04 so
2010-04-29 is shown for default documents.
Found here: http://ubuntuforums.org/showpost.php?p=3784265&postcount=8