Change happens. In fact, everything in the physical universe is changing. Note that refers to things in the universe, not the universe itself. Whether or not the universe itself changes is another question.
There are as many kinds of physical change as there types of energy: kinetic, thermal, chemical, electrical, electrochemical, electromagnetic, sound, and nuclear change (potential energy being related to kinetic energy). How do we measure change? We could simply compare one change with another of the same type.
But how do we measure change in general? Is there a framework in which all changes can be placed? The simplest answer is to adopt a single aspect of change, the beginning and ending of a standard change, as the unit of change. The question then is how many of these units of change does it take to measure a particular change (the measurand)?
A number of units of change is a rate of change. A standard rate of change would allow one aspect of every change to be measured. It would also enable a framework in which all changes could be placed if the standard rate of change generates a dimension of change that continues indefinitely.
What change would work well as a standard? It should have a regular rate of change. It should also have a clear beginning and ending, that is, it should be cyclic. Each cycle would constitute one unit of change.
The ancient standard was the cycle of days with the sundial as a convenient way to measure change. Over the centuries the movement of water, the burning of a candle, mechanisms of gears and weights, the cycle of a pendulum, and oscillations of quartz have been used as a standard rate of change.
Notice that I have avoided the words “time” and “clock”. There was no need for them. We could call a clock a change-measuring instrument. And time? That’s just another word for the rate of change measured by a clock.
Does time flow? If that means that change is always happening, then yes. But if the flow of time is supposed to be something intrinsic to the universe, that’s the assertion that the universe itself is changing. There is no need to go there to answer a simple question about change.