When did I installed this system?

Sometimes I wondered when I installed the system.  For Red Hat-based systems, here is a simple one: query for the 'basesystem' package

# rpm -qi basesystem
Name        : basesystem                   Relocations: (not relocatable)
Version     : 8.0                               Vendor: Red Hat, Inc.
Release     : 4                             Build Date: Thu 23 Sep 2004 06:01:44 AM HKT
Install Date: Tue 01 Feb 2011 05:26:36 PM HKT    Build Host: tweety.build.redhat.com
Group       : System Environment/Base       Source RPM: basesystem-8.0-4.src.rpm
Size        : 0                                License: public domain
Signature   : DSA/SHA1, Thu 06 Jan 2005 07:03:37 AM HKT, Key ID 219180cddb42a60e
Packager    : Red Hat, Inc.
Summary     : The skeleton package which defines a simple Red Hat Linux system.
Description :
Basesystem defines the components of a basic Red Hat Linux system (for
example, the package installation order to use during bootstrapping).
Basesystem should be the first package installed on a system and it
should never be removed.

The answer can be seen on the line with 'Install Date: '.

More information:This package contains no files, only used for initial system installation and can't be removed from the system afterwards.

# rpm -ql basesystem
(contains no files)