Sometimes I would need to find the date when the operating system was installed. Thou it’s difficult to have everything in shape with your inventory, if you have missed this part when an Operating System was installed, here’s a PowerShell solution to that.

Open PowerShell as admin
Type $osdate = get-wmiobject win32_operatingsystem
Type $osdate.ConvertToDateTime($os.InstallDate) -f "MM/dd/yyyy"

This will give you the date when the Operating System was installed

On the other hand, if you wish to go through registry yourself, browse to the location below

HKLM:\SOFTWARE\Microsoft\Windows NT\CurrentVersion\InstallDate

The result is in timestamp, so you need to convert it with this link.