Depends, I think of the universe as a flickering one :)

It has one perfect 'clock'. 'c'

You make all measurements under that clock, and we find that there is a scale in which we can't define it any further, the Planck scale. It is the time required for light to travel, in a vacuum, a distance of one Planck length. Going further down stops to make sense physically and mathematically, as main stream science defines it.

Planck time. "Before a time classified as a Planck time, 10-43 seconds, all of the four fundamental forces are presumed to have been unified into one force. All matter, energy, space and time are presumed to have exploded outward from the original singularity. Nothing is known of this period." And "As of May 2010, the smallest time interval that was directly measured was on the order of 12 attoseconds (12 × 10−18 seconds),[4] about 1024 times larger than the Planck time."

All of that makes a perfect sense of you think of radiation as a 'invariant' clock. As I see it we live by the beat of 'c'. And that beat happens at that Planck length described.

Then we come to gravity. There both you and me attach a importance, and a unique quality, to it. To me it is what makes 'SpaceTime' measurable, defining its three(four)dimensionality. And as 'gravity' bends the geometry of SpaceTime we find 'clocks' to tick slower, relative the 'far observer' that is. But not as seen if he would be in that 'frame of reference'. And that one is directly related to 'distance' as being frame dependent, as well as time.

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There is one more thing to notice. If the universe indeed 'flicker', then all observations we make can only be made while it's 'on'. So even if we wanted to define this as some 'smallest bit' it would still be a 'flow' to us, not 'bits'. We're only 'on' as the universe is 'on', if so. And the 'flickering' is the clocks invariant beat to me. There are other ways to see it too, a 'number space' for example where all motion is described by some 'number/symbol' changing a value. That one would give us a still universe, with only 'times arrow' defining what we see. And both can be put together, and are, in my mind :). But then again, it's my view.

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One thing that is special with gravity as a 'clock' is that all observers will agree to the difference. "An observer at the top of a tower will observe that clocks at ground level tick slower, and observers on the ground will agree about the direction and the ratio of the difference. There is not full agreement, as all the observers make their own local clocks out to be correct, but the direction and ratio of gravitational time dilation is agreed by all observers, independent of their altitude."

But when it comes to relative motion this change. "In special relativity, the time dilation effect is reciprocal: as observed from the point of view of either of two clocks which are in motion with respect to each other, it will be the other clock that is time dilated. (This presumes that the relative motion of both parties is uniform; that is, they do not accelerate with respect to one another during the course of the observations.)" From

Time dilation. That makes relative uniform motion a special case to me, and I'm still struggling with how to see it. An acceleration is 'gravity' locally, and can be defined from that. But, in the end, all motion confuse me. It has to do with the geometry, and gravity, but how? And it must have to do with 'energy, and the way it is distributed, displaced, whatever?

The universe is weird :)