I heard from a particle physics lecture that no one (till now) knows what charge is. We know the interactions of things with charges and more or less its connections with other things, e.g. through electrodynamics, but no one can say what charge is. It is the same with time. I think something is described by its characteristics and its connections with other things. Without these connections, we won't be able to describe anything. So, what do phycisists really mean, when they say they don't know what charge or time is, and what requirements have to be fulfilled in order to say that we know what something IS?