The past is everything that lies behind us, all we have accomplished, all we are; the future is raw potential, what we could become, what might happen. The present is where both sides of the hourglass, both existential behemoths meet in a single, unidimensional point; where it is decided which grain of sand goes through to shape the mountain below ever so slightly differently.
What is now?
Now is when a choice is made. Repercussion upon repercussion will pile onto the mountain of sand and the further away we go from this moment, the more different we will be compared to who we would be had that choice been different.