If you want to know to the exact time and date when your Windows PC was last shut down then you could find out easily by checking the Event Viewer. No special software needed for the purpose. There’s a specific Event ID attached to Windows shutdown and that is what you need to check.
A much, much easier way to find out is to open Task Manager, click the Performance tab, and look at Up Time. It tells you amount of time elapsed since last reboot or power down.
There is a small freeware utility called “windows elapsed running time” that will show the computer’s uptime when run. If you use the paramenters “-quiet” or “-hide” you can start the program showing only the tray icon and not the program window itself (see the readme file).