It is my understanding that Einstein got it wrong.
As a teenager he concluded that looking at a receeding clock would show the clock stopped. Time would have stopped, he would see a frozen instant.
He later realised this was wrong but had the enormous stength of mind, one might think - one might equally think willfullness, to persevere with his theory anyway.
It is wrong because all clocks moving at the speed of light relative to him are stopped, not merely retreating ones, for the theory to work. For example a passing clock, one he could reach across to and touch in his journey, would be stopped.