Well, a complex question.
In short, Time is merely how we reference change. Change is the underlying reality.
Change could be change of position (motion), or change of temperature, colour, density, weight, radioactivity etc etc. Change comes in many forms.
Time has 2 core distinct contexts in how it references change (one word, 2 separate meanings):-
Time is the dimension of change. In other words we calibrate / measure change rate using Time. Example, Motion (changing position) is calibrated in , say, miles per hour. The miles quantifies the amount, the per hour (time) calibrates the rate.
The flow of change. All around us and within us there is change. The universe is expanding, the earth is spinning, tides turning, you breathing, atoms vibrating, quarks decaying..all examples of Change happening. And Time is a non-specific collective noun for all this change. Another example of a non-specific noun is Traffic. Traffic is a non-specific set of vehicles. We give Traffic physical attributes…it starts, it stops, it flows. But fundamentally Traffic doesn’t exist, only individual vehicles exist. Time is to change what Traffic is to vehicles.
So, fundamentally, Time doesn’t exist. Change happens, and is reference-frame (including quantum) specific.
So all change happens at individual, i.e. reference-frame specific, rates. There is no universal Time.
And the past changes to become the present which changes again to become the future.
What is Time? 