What we call time is actually motion.
One day is one rotation of the Earth on its axis. One year is one revolution of the Earth around the Sun. If all movement in the Universe stops - time is frozen. If all movement is accelerated - time goes faster.
Velocity is a comparison of two motions - one is the movement of a body in space, and the other is the movement of the Earth. We call one rotation of the Earth on its axis a 24 hours. If a body travels a 1,000 miles as the Earth travels one rotation on its axis - that means the body is travelling at 41.67 mph. "Mph" means one mile per 1/24 of the Earth's rotation. If the Earth slows down - one hour is longer. If the Earth accelerates - one hour is shorter.
Why do we perceive time as a constant? Because we perceive the speed of the Earth's movement as constant.
Now, I will refuse to receive the Nobel Prize for this key scientific discovery because Tesla refused it too.
Speed is relative. If it were to slow or speed up, we would never notice the change, because again, it's all relative.
Time is a construct of man, God merely chose a moment for our existence.