What's the installation date of my Linux system ?

Well there is no "standard way" to know the installation date of you Linux system, so you could use one of these command :
- The idea is to retrieve the creation date of your root filesystem or /boot :

# sudo dumpe2fs -h /dev/mapper/VolGrp00-LogVol00 | grep "Filesystem created:" or # sudo dumpe2fs -h /dev/sda3 | grep "Filesystem created:" Filesystem created:       Mon Oct 29 11:44:51 2012

- Or by showing the oldest package installation date :

# sudo rpm -qa --last | tail -1                  
gpg-pubkey-307e3d54-4be01a65                  Mon 29 Oct 2012 11:45:27 AM CET

- Or the password information of your current user :

# sudo passwd -S user | tail -1 | awk '{print $3}'
10/29/2012

- And finally the creation date of the oldest configuration file/folder :

# ls -lct /etc | tail -1 | awk '{print $6, $7, $8}'
Oct 29 2012