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Under "System Software" the operating system should be listed with the install date under "Date Created"

Mac OS X Snow Leopard (10.6)

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.
   

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You can type vi /var/log/OSInstall.custom on the Terminal window.

Mac OS X Lion (10.7)

1. Open the console window in the Utilities folder, as outlined above.
2. Navigate to /var/log, then click on install.log
3. Scroll all the way to the top of the list, until you see a series of text that says "Install Mac OS X Lion"
   
4. The date next to the text is the install date.

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