The simultaneity of indirect experience is the association after logical reasoning. If two things A and B cannot be observed at the same time, but A and C, B and C can be observed at the same time respectively, and the elements in them are respectively simultaneous, then it can be inferred that some elements of A and B are simultaneous. Then C becomes the intermediary for judging simultaneity. The sun and moon are easy to observe and their positions change continuously, so humans first use them as intermediaries to judge simultaneity. Later, the judgment of simultaneity became more and more precise, and humans created a more easily observed intermediary - the clock. For example, if you tell me that you got on the plane at 1:00 yesterday, and I remember that I was eating at 1:00 yesterday, and then I believe that I was eating when you got on the plane, an association is established between these two experiences. The results of these logical reasoning can all be wrong and need to be checked.

Definition: The clock is the intermediary for judging simultaneous and sequential relationships.

The clock itself is not time, time is the relationship between the multiple direct observations of the same thing. The clock is only a medium for judging simultaneity. People often think of the clock when they say time, because they often observe the movement of the hands of a clock when they observe the movement of other things. In fact, the relationship between the multiple direct observations of anything is time relationship. 

Everything in the universe has the character of simultaneity or time relationship. So the motion of time is the motion of everything in the universe. The fast or slow change of time mentioned by Einstein also refers to the fast or slow change of the movement of various substances, rather than singly refers to the fast or slow of clock. When an object moves faster, its time slows down, which means that when its position changes faster, its other motion slows down. Other motions include changes in position between the parts inside the object, electronic motions, chemical motions, biological motions, etc.