In vacuum, no matter where you are or what you are doing or how fast you are going, the speed of light in your rest frame is "c" in every direction.
So suppose you are going at c/2 and go by another person who is going at c/4 and then another going at c/1000.
In your rest frame the speed of light is c in every direction. In c/2,s rest frame at the same point in space, the speed of light is c in every direction. The same for c/4 and c/1000.
At the same point in space, where you would think that a photon is a photon with a given speed, and that if you could go fast enough you could almost catch it- not so. It is always going c faster than you.
Yet at that same point in space, the speed of that same photon is only c in the rest frame of the point in space. Something has to give for that photon to be the same photon and traveling at speed c in both stationary and moving rest frames.
What gives way is time. Time is a variable, not c. Consider B & P in the rest frame of obj A:
A->0 B->c/2 P->c
Obj at rest Moving obj photon
Obj A sees B moving at almost the photon speed of light. On the other hand obj B sees obj A moving at almost the speed of light and obj B sees the photon moving at the speed of light in the opposite direction from A.
Consider the location of A, B and P at time 0 and time t in the rest frame.
xA=xB=xP=0 at t=0.
At time = t, xA=0, xB=ct/2 and xP=ct.
In A's rest frame B is always exactly between A and P.
But not in B's rest frame: P is moving in one direction at speed c, and A is moving in the opposite direction at less than c. B is not exactly between A and P. Special relativity derives the time that B experiences in order to make these contradictions consistent.
It would seem impossible for P to go at the speed of light, without A also going at the speed of light in the opposite direction. But like Moses told us, if you find the underlying principle that makes contradictions consistent, you will learn more about the hidden god.
The time B experiences is slower than what A experiences, so that the same photon is a distance ct past A, and the same distance ctprime past B. You would think that therefore tprime must equal t/2. Not so, for that would make A travel at the speed c away from B which is impossible. This is the contradiction solved by special relativity.
The solution is inconsistent with ordinary logic. Time is variable. For example, in special relativity, the photon does not experience time. In its rest frame t=0 all the time- which means all the time t of A's experience of the passage of time, P does not experience any change of time.
So to answer your question, the fact that the speed of light is always c in all moving frames means that time itself is a variable.