From a third-person perspective, one can observe that there is a continuous “when something first happens” in any material process (which includes both matter and energy), such as the moment when the bat strikes a baseball, or the position of the ball in flight at any instant of time. One can also have a sense of the mathematics of calculus related to the instantaneous position and vector of the ball. And one can have some understanding of the complex neural processes underpinning any moment of consciousness. In other words, as processes occur, there is a (metaphorical) “front edge” to them, that instant when something first happens.
From a first-person perspective, one can be mindful of experiences as they appear and then change, and in their changing, how they end in some sense. Here, too, there is that instant when something first appears in consciousness. There are many studies of perception related to this. More generally, this aspect of phenomenology seems inherent and obvious, that there is an ongoing “appearing” in awareness of the next perception, thought, image, desire. So, putting the third and first-person perspectives together, it seems that both objectively and subjectively, there is a “front edge of now.” Perhaps the entirety of time has already been made and our perceptions simply “slide” along it. Nonetheless, at every moment in that sliding there is always the next thing that is encountered – as something that is happening or experienced – and that next thing is in effect the front edge of the sliding. Check out this book for some more on the physics.