How to know OS Installation Date

Windows

WinXP Pro, Windows Vista, Windows 7 and Windows 10

Start > run > cmd > systeminfo

1. Open the start menu
2. search cmd 
3. open the command prompt application 
4. type "systeminfo" (without quotes) and hit the enter key.

Windows 8

1 - Go to the Search Charm How to Search in Windows 8

2 - Search "cmd".

3 - Select the "Command Prompt" App.

4 - Type "systeminfo" and hit "enter".

WinXP Home Edition

Start > run > hcp://system/sysinfo/sysinfomain.htm -> View the status of my system hardware and software

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

Mac

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.
   

Alternate Method

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

Icon

(:q to quit vi)

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.
   
   

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 Captain (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.