Time dilation

In the theory of relativity, time dilation is a difference of elapsed time between two events as measured by observers either moving relative to each other.

An accurate clock at rest with respect to one observer may be measured to tick a different rate when compared to a second observer's own equally accurate clock. This effect isnt a technical aspect of the clocks or it isnt from the fact that signals need time to propagate, but from the nature of spacetime itself.