A more robust version that tracks specific software and internet usage in addition to working hours. Security Task Manager:
For those who find Event Viewer too technical, several lightweight third-party tools simplify the process: PC On Off Time
If your PC On Off Time is sluggish, the culprit is usually one of the following "Four Horsemen." A more robust version that tracks specific software
View Last Boot Date/Time of a Windows Computer - TeamDynamix PC On Off Time