I want to know what are the minimal requirements for a physical system that must be satisfied in order for it to make sense to talk about time.
For example, there shouldn't be time in a system of a single free particle since nothing ever changes.
Another example - if we have two particles that move apart from each other, that distance between them is a measure for the time that passed since they left the origin. But if the particles are just rotating around their joint center of mass, the system always looks the same so there is no time. Can we have even more complicated systems that has no time?
Also, talking about a single particle or maybe even two is a bit not physical, in reality we cant have such an isolated system, so apart from theory, how would these principles apply in practice?