Time. We all know it. Or do we? There really isn't a good definition of time that doesn't refer to time. Relativity shows that time does not pass at the same rate from all perspectives.
Let's start with some basics. Time only goes forward. You can't go backwards in time. You can, however, stop in time. Photons don't recognize the passage of time. They don't recognize the existence of space, either, so it all averages out.
Why is this? Think of time as a vector. It has a starting point, a length, and it points in a direction. If you change the length, that's just changing the length, not making it go a different direction. If you rotate the vector, you just change the direction it's going. It's still going from now to when. It's just pointed in a different direction. Even if you reverse the direction, it still points from now to when. It's not going backwards. Change the starting point, and you're just moving the vector around.
You can't go backwards because the vector always points to somewhere else, or to nowhere at all. It can't point from it's starting point to somewhere before that point - there is no such place. Unless you want to speculate that the imaginary plane of existence counts as real, and that the length and direction of the vector are written as complex numbers. Then you might be onto something. But without that, in our real universe working with real numbers, it's not valid.
Why do time and space contract when something moves more quickly, or when experiencing a greater force of gravity? Because if they didn't, causality would explode. For more on that, go read up on general and special relativity. How does this work? Go check out my previous blog post, and some of the earlier ones on the subject. Summary - Spacetime is an energy field. All other energy fields (electromagnetism, mass, etc.) draw from this field. Momentum is a highly local (Planck length or less - my guess is 1/2 Planck length) distortion of the field around a particle. Velocity is the sine of the slope of the energy gradient. Time and space perception are the cosine of the slope. Notice that perceived passage of time is at ninety degrees to the passage of space. Thus, the fourth dimension is at right angles to the third. Nifty, ain't it?
Is there a universal time? Yes. There must be. Proof: Using relativity, photons do not experience the passage of time. Therefore, they do not move, they do not change, they exist in a timeless (literally) now. However, we can see that this is utter nonsense. Photons are created, move along predictable paths, influenced by the natural forces, change according to natural law, and eventually perish when they are absorbed upon interacting with a particle. (Except the ones that escape into empty space forever - a possibility.) If no time passes, how can the photon change? A photon obviously can and does change, or red and blue shifts wouldn't happen. So, there must be a universal tick affecting them as they move along, completely oblivious to what is happening to them.
Proof two - a particle entering a black hole experiences accelerations and gravitational shifts to perceived time. At the moment the particle reaches the event horizon, time stops for it. But the particle obvious continues on into the black hole, spending only an infinitesimal moment of 'real' time crossing the threshold at the speed of light. How can it do this if there is no time for it to happen? Universal tick.
Photons move at the rate of one smallest unit of space per one tick of the universal clock. Please note that this is movement, velocity, momentum. It is unrelated to spin, except that a whole (or, rather, half) number of rotations must happen per tick to keep everything even. No, I can't prove it, but it makes sense and seems to agree with known properties. (Particles spin faster than the speed of light. Sort of. It's OK, because they are really tiny, and they're not actually going anywhere. Divide really fast speed by essentially no distance, and it all evens out.)