Page Comparison

Mac OS X Mountain Lion (10.8)

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 Mountain Lion"
   
4. The date next to the text is the install date.

Mac OS X Yosemite and El

Capitan (10.10 & 10.11)

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 "OSInstaller" 
   
4. The date next to the text is the install date.