The Cloud is a neighborhood

For different reason, you might want to know when Windows installed. Luckily you can find this information other places then checking out the date on certain system files that was created/modified and stamped at the same time as when you installed Windows. The file date stamp method does not in calculate that files change this information during upgrades etc. during the lifecycle of the installation  In registry, you have a Key to help you with this information. HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\InstallDate

There is no doubt that as a company you should have setup a way of signing all the PowerShell script you are intending to run in your production environment. But as we all known there will be situation where you want to run unsigned code, or maybe you are testing things out before you are ready to sign them.