A definition of time, from a scientific perspective, is an important question and undertaking.

There is a very definite problem with ‘time’ being a physical quality or quantity. As others have asked – ‘what is physical about time’?  A particular issue is: if it is not a physical quantity – how can it be represented by a 4-dimensional spacial/physical model per General Relativity (GR)?

Proposed definition of Time as a quantity: “Time is the measure of change.”  It doesn’t matter what change or in what direction the change occurs. Any measurement of time is a measurement of some change in physical objects.  This is not a physical quantity.

The measurement of change is easily modeled via a linear variable – however this variable is directly related to the change being measured.  There is no ‘a priori’ reason that the variable measuring change in one situation is the same variable used in another situation (some consider there to be seven ‘arrows of time’:  “e.g. the CP arrow, the thermodynamical arrow, the psychological arrow, the radiation arrow, the measurement arrow, the cosmological arrow, the Gödelian arrow.” [thank you Erkki Brandas, from a Researchgate thread]).  That they are all the same is a presumption of science today and underlies the ‘universal time’ concept Newton believed in.  Certainly we have a ‘sense of time’ which strongly suggests a universal time and universal simultaneity – however GR strongly suggests this is not actually the case and time is relative to the observer.  So maybe we need to question whether all these ‘arrows of time’ – all these measurements of change are or should be modeled by the same single variable.

Note that if all measurements of time cannot equate to a single linear variable, then they also cannot equate to a single physical dimension – as per our current interpretation of GR.  This would indicate something amiss with the theory of GR.  Note this possibility does not have to change any of those equations, just our interpretation of them – in particular what the 4th physical dimension in the equations represents.

Is there simultaneity in the events occurring on all levels of scale?  When a billiard ball collides with another one – are all actions of this event, on the macroscopic, microscopic, atomic, particle, solar, galactic, super-cluster levels simultaneous?  I think we assume they are – but how can we tell? Can our current models actually answer this question?

If all actions stem from the smallest level – won’t there have to be a time-lag between what occurs on the smallest level and the effects on other levels caused by those on the smallest?  To turn this question around – if everything related to an event (eg. the billiard balls) on all levels of scale occur simultaneously, how can there be cause at one level and effect at any other, as there will be no measurable change (time) across levels of scale?

Scale and Causality:  If there is any possibility of cause and effect across levels of scale, then there must be some change across scale, some measure of time across scale, and therefore the possibility of movement across scale.  Such a conclusion would indicate scale is another physical dimension and our current model of ‘space’ is inadequate.  What if this dimension is the one involved in the equations of GR and not the non-physical concept of ‘time’?