Often that is attributed to as-if said by Einstein, but he only said that past, present, and future are illusions, because "now” is relative to an observer.

Having said that, his time as 4th dimension of his spacetime surrogate, is a figment of his imagination, a mental tool, good for doing math (like applying Pythagorean theorem combining time projection/leg and space projection/leg, and for applying Lorentz transformation to spacetime).

You may continue to Physical Answer to “What is Time?” right away or stay here for a brief explanation of what is real/objective (like time dilation) and what is not (like observer or some GR deductions), using Newton’s Third Law Is Violated: Action ≠ Reaction! as an example:

Recently Dr. Vivian Robinson did all the complex math Einstein did (using field equation in GR – General Relativity) to get the exact correction for the Newton’s inverse square law for gravity: F = G×m×M/(R²×D²), where D is time dilation near the observed object. And when we explain the meaning of such correction in context of Newton’s 3rd law, we start with “F×D² is the same for Action & Reaction” – which is still explained using an observer:

But then we come to two interesting conclusions:

1) F~1/D² dependency is actually true for any force, not just gravitational (electrostatic for instance), beyond GR derivatives.

2) Then we end with rewriting the Newton’s 3rd law as F1/F2=(D2/D1)² with the middle man (observer) eliminated:
D1 — time dilation we observe around the Sun,
D2 — time dilation we observe around the Mercury,
D2/D1 is time dilation between time on Mercury and on the Sun, irrespective of the observer’s whereabouts. That means the observer concept does not really matter since ratio D2/D1 stays the same for any observer: on the Earth, on the Sun, on Mercury itself, or with any other observer’s location.

Continue 👉 Physical Answer to “What is Time?”