According to the special theory of relativity, the concept of simultaneity is relative, and the perception of which clock is running slower or behind the other depends on the observer's frame of reference.
Let's consider an example to illustrate this. Suppose there are two observers, A and B. Observer A is stationary, and observer B is traveling at a high velocity relative to A. Both A and B have clocks.
From the perspective of observer A, who is stationary, it appears that observer B's clock is running slower. This is because time dilation occurs when an object is in motion relative to another object. The faster the velocity of B, the more pronounced the time dilation effect, and the slower B's clock will appear to A.
On the other hand, from the perspective of observer B, who is moving, it appears that observer A's clock is running slower. This is because, according to the principle of relativity, both observers are entitled to consider themselves at rest. Therefore, from B's point of view, A is the one in motion, and A's clock experiences time dilation.
It's important to note that both observers are equally valid in their descriptions, and there is no absolute "correct" answer to which clock is running slower. The perception of time dilation depends on the relative motion between observers and is a consequence of the relativity of simultaneity.