Toward the end of his life Einstein had a conversation with Rudolf Carnap (ca 1953-54):
“Einstein said that the problem of Now worried him seriously. He explained that Now means something special for men, something essentially different from the past and the future, but that this important difference dies not and cannot occur within physics.”
Einstein was still struggling with counterpoints made by Bergson in their famous 1922 debate on time and the meaning of duration.
Neuroscience is just beginning to give us more insights into Now and I am reasonably confident that we will find solid (satisfying) physical explanations for human temporality (more so than those of Bergson, Husserl, and Heidegger). But this will not remove the personal mysteries of attention and being in the flow.