Record time stamps and time intervals by pressing the on/off button.
Send data to your personal server, and generate full history traces and/or statistics, for as many devices as you want.
This app is very generic, and can be used agnostically to come around various problems like tracking time spent by users/employees doing something specific. As the button is in their control and depends on their consent, it's up to you/company to create a policy making it their best interest to use it properly.
Remote server is not provided. URL can be configured in app settings. Devices are identified as displayed in Settings screen. Add a database to store data, and generate history traces or statistics. See https://github.com/PAPLabs/OnOffPost-Server for a sample server implementation.