How To Check Operating System Installation Dates

Windows : 

• Open Command Prompt ( Run ) using  Windows + R or Direct Navigation  and enter systeminfo
  

Mac OS X Snow Leopard :

1. Open Macintosh hard drive on desktop, find Application folder and open it.
2. Scroll Down and open Utilities folder.
3. Open the console window in the Utilities folder.
4. In the console, expand the directories as shown below, find and click on OSInstall.custom, you will find the OS install date as shown below.

Linux:

1. run this command  sudo passwd -S sys | tail -1 | awk ‘{print $3}’ 
2. And the output will be like this 03/11/2013 ( This is your installation date )