What is Change? Change can be defined by pointing at processes. By pointing at different parts of the process and comparing them. If the parts are the same, then no change has occured. Processes are defined to evolve in time. Time is connected with a cyclic process. It is the number times the cyclic process has repeated. Now this might sound circular. How can you define time on a base that needs time in the first place? If a process is defined as a situation evolving in time, how can we use a process in the definition of time?
The answer lies in irreversable and reversable processes. A reversable process is used in the definition to quantify the irreversable processes (or another reversible process). A pendulum is a cyclic reversible process (in the ideal case) and time is defined as the number of times (how appropriate!) the process has shown the same face. This definition of time is not applicable in defining the reversible process itself. You could
compare a reversable process with another reversible process, but what's the use? A sinusoidal, reversible process can be a function of time but that time is again based on another reversible periodic process.
There are few truly reversible processes. The reversable process time refers to is that of an ideal clock and a realization is difficult, if not impossible. Of course there are 60 billion watches with a strap to sell, and an atomic clock is kept in isolation to compare all other clocks to, but an ideal clock is impossible to construct and will always stay an idea. A real clock will always show failure of a constant period, no matter how closely approximated.
It's this ideal clock that serves as the background of theoretical processes, when theoretically described and which are measured by a theoretical observer. Like the distances this theoretical observer measures refer to an ideal unit of distance which can only be approximated in the real world, like a standarized meter kept at constant temperature or a defined as the distance that light travels in a standarized amount of time (which shows the connection between space and time), which only shows that the meter is not an exact realization of the ideal, since the clock that measures the time needed can never be ideal or reversible.
So for change we need different situations which are part of an irreversible process. The parts of a reversible process that repeat themselves are used to compare two situations that are separated in time. Reversible processes induces the notion of time. They exhibit the reversible cyclic behavior relative to which the evolution of irreversible processes (or other reversible processes) is measured.
Irreversible processes are the key to change. They induce the notion of reversible processes, which induce the notion of time to measure the rate with which parts of irreversible processes become different or indicate if processes are reversible. Now you might ask if parts becoming different doesn't beg the question. I think it's clear though. If you define becoming as distances between particles varying, there is no circularity.
If you reverse time, which is the same as reversing all momenta of all particles, the clock will just reverse it cyclic motion, and no difference with the forward clock will be seen, as the clock is based on a reversible cyclic motion. The numbers the clock is attached to though, will be decreasing instead of increasing (it always amazes me why the counting before a rocket's ignition is backwards, though time zero, as well as ground zero, are regarded special; as if it all starts with the ignition of the rocket). This is the solution to the question why time is not reversible. If you reverse time the numbers decrease, counting back to time zero. This implies though that there is a beginning at infinity, which is an impossibility. On top of that the boundary conditions would have to be finely tuned with infinite precision, but since infinity has no boundary, this can't be realized. Locally we can reverse time, but globally this is impossible.
If space, time, and charge are reversed, then according to the TPC theorem in physics, processes look the same, a strong indication that a mirror universe must be there.Around t=0 there obviously was no clock present. But we can look at that state as a clock going forward and backward around time zero.
To experience change we must have a memory of past parts of processes. You compare the perceived situation with a memorized one and change will appear naturally. Of course you can have a change of heart and think that no fundamental change has appeared, which can diverge you to different conceptions of change. Is there a change in general? A change that is valid for everyone? No, there isn't. One man's change is the other's static.