Operating system time and Date
 

please tell me how do i know when the operating system is installed? where can i see the date and time of the installation?

If you are using one of the Debian types you can get some info by looking at /var/log/ then either cat or less installer. If you are using Gentoo you can get some info /var/log/ the cat or less emerge.log as for any of the other Linux distro's I would guess you will find some info in /var/log/ as well. It may help if you tell us what distro you are using.