1 second = 3x10^8 metres * sqroot (mass / energy)
Time (the left hand side above) therefore is relative (nothing new there) and depends on a constant of distance (ie. space), as well as the mass and energy of ..... well what exactly?
Lets think again about the Minkowski Space Time model. The above equality suggests that time cannot be simply attached to space as a dimension if it has a co-dependency on mass and energy. At least, to do so introduces an ugly complexity involving mismatched 4-vectors.
Time must be a property of mass and energy that is expressed in a constant unit of space. We can be certain that time cannot be expressed as a property of empty space alone, as the Minkowski model suggests. Time requires something to be in it - mass, and its energy - motion. Without mass, energy and space, time cannot exist. Thus the Minkowski model (simplified from the complex space-time model originally proposed by Henri Poincaré) is an imperfect model for understanding time.
If, as Einstein established, mass is itself a property of energy, then time is also fundamentally a property of energy. But this definition is circular because our common units for energy (joules, electron volts (etc)) have time dependencies. That is why we have trouble understanding time.
But what are the mass and energy and spacial constant referring to anyhow ? We could suppose that a time of 1 second of a body in S1 space (moving directly away from an observer) is defined by the mass and energy of the body and a universal constant of space. Does this help ? This is simply analogous to the movement at constant velocity of two reference frames in Special Relativity.
Defining time in S2 space by squaring both sides may clarify:
1 second^2 = 3x10^8 metres^2 * (mass / energy)
What this says is that time of an observed accelerating body is constant over an S2 space but varies with its mass and energy. An analogy is gravity on the surface of a planet - ie. general relativity.
It is prudent to remember here that Einstein's mass - energy equivalence was relativistic, not absolute. Time based on mass - energy equivalence must also be relative.
Yet it is tempting to suppose time can be defined absolutely. Indeed, why shouldn't this be so? Time is after all a metaphysical construct invented by primitive man to describe or understand motion. It doesn't actually exist other than in our minds. We've inherited this erroneous thinking from even before it was believed the earth was flat.
But Einstein's hitherto unchallenged yet mathematically incorrect claim that "we are moving through time at the speed of light" comes tantalisingly close to the essence of time. Time is motion, yes, but in a qualified sense only. Einstein himself shows us in indisputable clarity that time is a mere artifact of mass, energy and space. Wherever these three elements are combined, time exists. Wherever one is missing time does not exist.
So how do we connect with all this?

If I may paraphrase from "The Matrix": Then you'll see, that it is not us that moves through time, it is only time that moves through us."