Time is man's greatest invention to date.
But it might be more, it might be a real thing, a discovered thing.
Certainly processes begin and end, there are markers of change. When we start to infiltrate time more deeply though, there are questions about when beginnings begin.
Look at this drawing. Imagine event P and Q occur at the same "time." The photons which carry the information begin propagating out into spacetime. The axes aren't labeled, but x is space and y is time, hence the cone shape, light will really propagate in a sphere when unobstructed.
Now event P and event Q are nonexistent to each others' "senses" until their time cones intersect, which is at the shaded area near the top. This is important because it shows irrevocably that information which exists is also impossible to access.
Funnier things happen if you switch the grid which P and Q live on, and pretend to live on different planes. P can appear to occur before Q and vice versa, though we know I've set it up that they are simultaneous events.
Unfortunately, we don't have access to the original relative frame. There may or may not be one, but without one, it is impossible to know which set of frames we are on, and therefore how events are ordered.
Combine this frame talk with Einstein's observation that photons (light) travels at the same speed in all frames and we come to my potentially original thought:
Light appears to have a constant speed for all relative frames because the process of photon generation is intrinsically equal to the smallest physical unit of time.
This makes me think that perhaps the true relative frame which is at "absolute" rest is the timeless frame in which all photons, past present future, are created. It makes me think that photons are the true timekeepers.