UNIVERSAL "ULTIMATE" TIME AND QUANTUM THREAD NETWORK TIME
There are actually two different types of time.

1) Universal Time: If you removed all matter and energy from the universe (that means remove all particles and substances, energy is a vibration on a substance or particle) and were left with only an observer (for instance Einstein) and his pocket watch... would time still exist?
Yes, that is Universal Time in a null universe - void nichts space. This would be unchangeable constant flow.

2) Quantum Thread Network Time: Space is NOT empty. There is an all encompassing lattice-type quantum thread network in otherwise empty space.
It is gravity centered - Einstein called it Space-Time. Otherwise known as the Fabric of Space. (yes, the fabric of space must actually be made from something)
A good 2-D model would be something like a spiders web made of the finest web-silk-thread filament. Now imagine a 3-D web.
It is made from individual yet connected quantum threads and of course the web has tension on it (that's were gravity gets its pull). The speed vibrations travel through the web is the speed of light (light is a just a vibration travelling in a quantum thread network)

Fabric of space is a gravity-centered thread tension network. (never ruled out)

Here is a regular thread tension formula...

Tension = velocity squared x mass / Length
Vibrations traverse network at c
Plug in c, rearrange

TL = mc^2
| -- inch -- |

Also the source of energy

Thread length L and mass m are unchangeable constants.
Variable thread tension T will also vary the speed of light c.
Speed of time (as we know it) is also variable.

The thread network tension is the universal cosmic speedometer.

Everything is completely regulated by the thread network tension.
Thread network tension is completely variable.
Tension determines speed.
If the tension goes up: gravity goes up, and so does the speed of light and everything else with it.
That includes any type of measuring device, the speed your brain is working and time itself.
Increase or decrease tension and it changes everything along with it, that's all electro-magnetic phenomena, vibrations... everything.
It's like being a character in a movie and you don't know the speed the projector is running... fast, slow, stop, start... you don't know.

Net effect: you might not notice anything.

But if you could remove yourself from the projector, take a step away from everything and take a look back, you would see the speed everything is happening.
If the tension has been changing over billions and billions of years... this would be readily visible as red-shifted light.

I'm talking about something completely independent from everything.

Like H. George Wells sitting in the time machine.

Anything Einstein said or proved is completely irrelevant to George.
George can see things happening at all kinds of different speeds... and everyone else is completely unaware of it.
The same thing can be happening without a time machine.

There is no way to know the speed things are really happening.
If everything is happening very slowly so is the way the brain sees it.

You have to think a level or two deeper to understand and most people can't.

There was an episode of STV with a planet with fast time. The inhabitants of the planet were completely unaware they were moving so fast. Same thing is true for anyone / everything... there is no way to know your true speed (unless someone / something comes along with different speed) - but you still won't know the true speed - you will only know your speed is different than someone else's.
For true speed you have to remove yourself from everything (the thread network ), take a look back and hope there is a noticeable change in speed in the thread network time (or figure out how to negate the thread network time your body might still be using).

● "Universal Time" is like a guy running a movie projector. He can tell exactly how fast the movie is running and the movie has no affect on him.
Universal Time is completely independent from everything and anything.
Universal Time would be happening in literally empty void nichts space - completely null, naught, vacuous.
Neutrinos floating around in space before the fanric-of-space was created are in universal time.
The neutrinos floating around now are unaffected by fanric-of-space and also still in universal time. ● "Thread Network Time" would be people in the movie. They can't tell what speed the movie is running because they are actually in the movie - they are made from movie film.
Space has the thread network in it (that is what Einstein calls spacetime). The speed of light is the speed of vibrations traversing the thread network, and of course it is the speed everything works - including people and all matter.
Thread network time is an emergent property directly related to the speed of thread vibrations. Got it? Empty space is completely empty, but there is something in it so, it is NOT empty.
That explains the famous claim: "Empty space is NOT empty" The Flow of Time: Gravity is the main reason time can only flow in one direction.
Gravity is caused by a lattice-type thread network, and network tension can only pull things together.
That cannot work in reverse. A couple other things related to that are also happening
We are immersed in quantum thread network time - that is like a movie.  
The speed of "time" for us changes relative to thread tension: TL = mc^2
Universal time - is the guy running the projector - it is completely unaffected by anything.

3) Planck Time? (this might take a while)
"A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it." -- Max Planck

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THE SUN IS THE SAME IN A RELATIVE WAY
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This is interesting... if you could put everything back the way it was in the past, has time itself also reverted to that period? That would mean "time" is just the way things are arranged.

If I had a time machine, I (just myself) could travel back to 1947 and drive out to Roswell in a 1947 Chevy and see what really crashed there.

But what if I use the time machine and transferred the whole planet back to 1947? It would leave the planet exactly as it is now but in universal time it is 1947. I could still drive to Roswell in a modern car and the UFO crash would still happen because it originated from off-world.

Taking this one step further... I transfer the whole Universe back to 1947.
Everything is exactly the way it is now but Universal time is 1947. Now the crash does not happen. Even though it is actually 1947... the arrangement of stuff in the entire Universe is 2021