I am having a bit of trouble with understanding a basic point re the concept of relativity of simultaneity. I get the point of observers seeing events at different times, due to the passage of time of light travelling to them from the event. But I don't understand why this precludes events being known as occurring simultaneously. One might not actually observe this, but knowing the speed of light, each observer can gain an understanding of when the event actually happened by deducting the time that the light took to reach them. Thus an event at C may have taken 1 week to be seen at B and 2 weeks to be seen at A, but both A and B can calculate that it took place at the same time. I am clearly missing something.