The laws of physics run the same forwards and backwards. A broken cup never reassembles. Both of those are true, and the gap between them is where the arrow of time lives.
Ask what a clock measures, and the answer is that it counts. A pendulum counts swings, a quartz crystal counts vibrations, an atomic clock counts the oscillations of a caesium atom. Time, at bottom, is counting. In the foam there is a smallest thing to count, the displacement event, one bubble, one void, one B + V = D, and a shortest interval in which one can happen. The world advances one step at a time, and the fastest it can pass a disturbance from one cell to the next is the speed of light. That limit you learned about is really a fact about time. It is the frame rate of the world.
Take any fundamental process and film it. A photon scattering off an electron, a wave moving through the foam. Run the film backwards and you are looking at another process that obeys exactly the same laws. Nothing in the equations marks one direction as later than the other. The equation that governs a disturbance in the foam is time-symmetric: play a recording of a wave spreading and a recording of a wave gathering, and neither one breaks any law. The deep machinery, the part that gives the particle masses and the fine structure constant, does not know the difference between past and future.
Film a cup falling off a table. Forwards it shatters. Backwards the shards leap off the floor and assemble into a cup that hops onto the table. One of these you have seen. The other you know, instantly, is impossible. Yet every single collision between every single molecule in that film is perfectly reversible. The impossibility is not in any of the parts. It shows up only in the whole. Something breaks the symmetry, and in the foam one thing does.
When a displacement event resolves, it leaves a mark in the void structure of the foam, and that mark cannot be taken back. There is no operation in the framework that unwrites a void, because unwriting one would violate the axiom the whole world is built on. The wave can come and go. The imprint stays. So the motion of the world is reversible, but the record of the world is not. Time's direction is not the direction things move. It is the direction the record grows.
The past is the part of the world that has been written. The future is the part that has not. The present, that thin bright edge we can never quite catch, is the write-head itself: the one place where the foam is actually setting down a mark that will still be there when every star has gone out. We live on that edge. We have never lived anywhere else.
Two parts of this are framework physics, not a story laid on top. That the foam has a smallest tick, and that its underlying wave motion runs the same both ways, are properties of the equations. That writing a record into the void is the one step that cannot be reversed is the framework's account of why the large-scale world has an arrow at all, and it is the same mechanism it uses for measurement. What a written record feels like from inside a life, memory and forgetting and the thinness of the present, is a reading, offered as one careful way to hold the physics rather than a result anyone has proven. The full treatment is the forthcoming companion book, The Record: Time and Memory in the Foam.