Time is experienced (sometimes measured) change.
The laws of physics explain how things change over time. Δ^∞ (change over time, infinite in all directions, at all scales, forever) is the formula for Actuality.
Time is experienced as a sub-set of Actuality that changes at a speed that is compatible with our embodied frame of reference, the clarity of our senses.
The past is remembered experience. The future is anticipated experience.