cmd.exe:
SystemInfo | Find /i "Install Date"
Powershell:
(Get-CimInstance -Class Win32_OperatingSystem).InstallDate
regedit.exe:
HKEY_Local_Machine\System\Setup\*.*
*Source OS (Updated on 3/5/2020 13:50:25)
*Source OS (Updated on 11/21/2019 10:51:24)