How to find when my os was installed?

[root@sysadmin Desktop]# last |grep wtemp

[root@sysadmin Desktop]# last

 [root@sysadmin Desktop]# lsb_release -a
LSB Version:    :core-4.1-amd64:core-4.1-noarch:cxx-4.1-amd64:cxx-4.1-noarch:desktop-4.1-amd64:desktop-4.1-noarch:languages-4.1-amd64:languages-4.1-noarch:printing-4.1-amd64:printing-4.1-noarch
Distributor ID:    CentOS
Description:    CentOS Linux release 7.1.1503 (Core)
Release:    7.1.1503
Codename:    Core