Physical existence alters. We know this because when we compare physical input received, that reveals difference. The rate at which any given physically existent state alters to the next in the sequence can be timed, the unit of this measuring system (timing) being known as time, a duration. Either that can be effected directly, ie by comparing one change sequence with another. Or it can be effected by comparing to a conceptual constant rate of change. This is achieved with timing devices which 'tell' the time (ie the constant). In order for the measuring system to function, all devices are, within the realms of practicality and sufficiency of purpose, set at the same point and maintain the same rate. The mechanisms used in these devices vary, for example, in a quartz device the time is being 'told' by crystal oscillation. But the output is converted into a common language (eg days, hours) in an easily readable form (eg hands on a clock). The language is fossilised, reflecting that the first timing device was earth movement.
Timing is a measuring system which enables the calibration any given rate of change in physical existence, against a conceptual constant rate of change (ie a common reference). It concerns a feature of the difference between any given physical realities, not any feature of any given physical reality. Since it is measuring the rate at which change occurs, it is not reversible, because change is not reversible.
In the first section in 1905 Einstein (following Poincaré) explains how timing works. This is incorrect. He and Poincaré failed to understand that the system is really referencing to a common denominator. That is, the timing devices must already be synchronised to a 'common' time, otherwise the system is functionally useless (variations due to the aequacy of construction of any given device are irrelvant to this point). This reflects a fundamental flaw in Einstein's thinking, ie the failure to differentiate what occurred, from the light based representation of that occurrence, which is what we receive. Which means there is always a time delay between physical existence and receipt of a physical representation of it, the timings of the two events are different, that being mostly a function of distance.
By presuming, incorrectly, observation (ie receipt of light) as being equal to occurrence, observational light, and the consequent time delay, is eliminated. So the light which Eintein refers to is being used as a timing mechanism, t becomes c, which is why it must be constant. Whereas observational light approximates a constant. The duration differential, which reflects physical existence, is then reintroduced with the mis-use of x=vt. Distance has no duration, because it is a difference between entities as they physically exist at the same time, there is no distance between something which exists and something which does not.
But distance can be expressed, conceptually, in terms of duration incurred. The concept being that instead of expressing distance as the fixed spatial quantity which it is, it can alternatively be quantified as the duration which would have been incurred had any given entity been able to travel along it, either way, which it cannot. But it must be understood that there is no duration as such, this is just an alternative to, and the equivalent of, a spatial measure, ie a singular quantity. Distance is always unique, since it reflects a definitive physically existent circumstance at a given time.
Einstein failed to understand this, and furthermore, by calibrating distance in terms of consecutive timings (ie the differential in travelling the distance either way) he reintroduced, approximately, the duration which he had incorrectly eliminated. It is approximate because the real duration is dependent on observational light in any given real circumstance, whereas the difference he derived was the function of a constant. And in order to measure distance in terms of duration, Einstein used light, which in the context of the circumstance as he presumed meant that it was effecting the role of the time constant, not the physical entity which involves the capture and transmission of a representation of what occurred, ie what is normally referred to as light.