The reference photon
When we see a clock face, and discuss motion in the very familiar language of Time, hours minutes, seconds, miles per hour or kilometre etc, the very familiarity we have with all the terms and our very 'natural' use of them, makes it seem obvious that we can compare motion to another 'thing' called time.
Indeed many, if not all, major projects like running a business, a railway line, safely would be very hard without the concept and language of time. And organising something like a successful spaceflight and rendezvous would be virtually impossible.
But if we want to make the most complete and simplest working model about what does and what does not actually exist in the universe, we should perhaps bare in mind that a 'clock' is just a set of motorised hands that give a convenient and accurate example of something 'just moving now' at a constant, - and thus very useful - 'rate'.
Calling the distances or angles between the numbers on the dial 'hours' is useful but does not prove that 'hours' and time really exist. Note also that no 'clock' proves that 'the past' is created and exists, or 'the future' exists and events, random' or 'predictable', 'come out' of it.
All a 'clock' proves is that 'motion' exists, and we can compare the motion of a motorised hand to the motion of a car or runner etc.
The key area of confusion I am trying to clarify here is that if we casually use the terms 'clock' and 'time' then we are implying that time exists and is measured by clocks... Which may seem fine - which is the problem! Because in implying time exists, we are also implying (but not openly) that there is a 'past' that is created or added to as things happen, and 'time passes', and implying that as things happen, we all 'head into an certain, or possibly certain 'future'.
Or, buy casually using the word 'time', and seeing how the 'concept' works, we are implying (but not openly) that perhaps 'block time', or 'growing block time' exists, and perhaps there is a 'dimension' 'T' which may be in a sense like a dimension, or direction 'x,y or z' - such that 'in the future' or 'in the past' is similar to saying 'over here or over there' - which is the basis for Einstein saying 'For we convinced physicists the distinction between the past, present and future is merely a persistent illusion".
The point being, by casually using the word and tools of 'time', without being open to what we are also implying, we make the jump from...
-"hey, comparing two moving things gives us some very useful maths"
to
-"hey, theres an extra dimension to the universe, involving an unimaginable and intangible 'temporal past' and 'temporal future', all of which may be here 'now'. And through which we might theoretically be able to 'time travel' in the same sense that we can go from here to there'.
Which is a massive set of assumptions to casually imply, buy just using the words 'clock' and 'time' ,all without openly declaring any of them at the outset. So what is a 'clock', if not something that measures time?, Let's look at the simplest aspects of the idea first.
What is the ultimate or simplest 'clock' then ?
If there is no time, then you might ask, how can we understand and work with all the movement and change around us without using Time?. My answer to this is that we DO use Time. we just remember that time is just a useful concept and set of words to describe and compare the motion of different things. And we also in particular remember that concepts like the past and the future don't actually relate to things or places that actually really 'exist' in any way at all.
in other words, use time, but don't start imagining that it really exists and flows, or waste your energy and effort, wondering about the possibility of traveling into the (nonexistent) past or future.
If you want to be very clear about what you are actually doing in any work or equations etc that involve Time, you can use the idea of a 'reference photon'.
If you imagine any equation using 'T' in seconds etc within itself you may relate that 'T' to a second hand ticking around a dial - and thus to Time.
Alternately, you can imagine a very long track, marked at 300,000km intervals 1,2,3,4 etc
If you then consider a photon flying along that track you can see how the 'reference photon' would correspond to the 'second hand ' on the clock. We use this set up and in particular a 'photon' because that is basically the ultimate 'moving object' (Tacyons etc being to esoteric)
In this way you may see that while we think we are using 'seconds' and that in some way the fact our equations relating to moving objects works 'proves' that objects move AND that seconds ie TIme exists in some way - if fact these equations only prove that 'objects move' - be the object a falling stone or the 'reference photon'.
You can also see that while this system would work perfectly in replacing the numbers and concepts we see as 'seconds' - the reference photon is just a thing moving along a numbered track, and at no point does it prove that there is a 'temporal past' behind it, or that there is a temporal future ahead of it. -it just proves that things can exist and move, and that we can compare moving things for convenience to make sense and calculations about the word.
(this is all true of course for a photon in one of Einstein's 'light clocks' which are same as the above set up but trapped between 2 mirrors for convenience, but it too does not prove that there is a 'temporal past' behind itself , or that there is a temporal future ahead.
Because 'time' does not exist other than as a concept and mental tool, there is no need to replace time with something else - we just keep using it as the tool that it is, but realise it is a tool for comparing motion 'now' ( to use that redundant word), and that the tool does not prove that anything other than now exists.