You are asking about time, a very difficult thing to grasp. According to Einstein it is relative. Your time is not my time, and vica versa. Time seems to depend on the observer... But lets forget about that, to keep things simple.
I think you can compare it to a computer. More and more scientists come to the conclusion that the universe is actually one big computer, E.g. My Big TOE written by Tom Campbell, a former NASA physicist, Programming the Universe by Seth Lloyd who proposed the first feasible quantum computer, and others).
A computer only has it current state. The past states, or possible calculable future states do not exist. The future states only exist in potency but haven't been actualized. And the past states don't exist anymore either. With every CPU tick, its state is altered. Nowadays a computer is very capable of rendering some nice graphics, 3D if you will. A car moving on the screen seems to have motion, yet it's movement exists of a sequence of positions projected on the screen on after the other. If you go along with the universe being one gigantic computer, then particles moving is the same. It is granular, not continious. Hence planck length and planck time are the like the pixels on a computer screen. And the speed of light is the max frequency of the CPU of the computer.
If the computer isn't observed, its state is undetermined, it is a cloud of chance (like Schrodinger's cat, or decaying atoms). When you probe its state, the wave function collapses to a state, which is always in the now, in the present moment, the moment of observation. Hence that is the only state that exists. But as soon as you would be able to observe again, that state is not actual anymore. Because a new state could be determined.
So assuming a computer can only calculate now, it can not calculate in the future nor in the past I would go along with your statement that things change in the now. Maybe that is due to consciousness observes in the now, just like the particle-wave duality double slit experiments.