Of moonlighting by the day: Is one job enough?

After working for years, I realized two more things. That the concept of a full-time employee is not really that clear to me. Or to more people for that matter. After all, whose time are we talking about? If one person has more time in a day than is required for the job he/she is assigned to do and the outcomes needed for that, do they not own that time and can they not choose how to use it, not just for leisure but also for other work?