I got lost somewhere in this deduction that time is not simultaneous/equivalent for different reference frames. There are two points A and B that are distanced apart from each other along a railroad (the railroad will act as the reference body). Point M is the midpoint of segment AB. While testing the simultaneity of two lightning strikes (one at point A and the other at point B), an observer stands at point M to observe whether the strikes are simultaneous. The observer finds that they are indeed simultaneous. If we were to shift the reference frame to instead a train moving along the railroad towards point B, we would have points A' and B' (both points on the train), in which A' would coincide with A and B' would coincide with B' at the moment the lightning strike. If there was an observer at point M' (on the train), which coincides with M at the moment the lightning strike, then because the train is moving with a velocity in the direction of point B, the observer at M', unlike the observer at M (for when the railroad was the reference body), would see the lightning from point B' (which coincides with B) before the one from A'. Time is not simultaneous in both frames, thus the time in one frame is not necessarily equivalent to the time in another. What I don't get is what they mean by time in different reference frames not being equal. Would time be analogous to velocity in this case?: If there were two reference bodies, one stationary (K) and one moving with a velocity (K'), and there is a moving object P, the velocity of P with respect to K is different than with respect to K', though the overall behavior of the phenomenon can be modeled so that its nature with respect to K does not contradict its nature with respect to K'.