Now is the present moment, but we also cheat a bit and use memory.
For example: now is such a micro moment that we add to it our memory of what has become past. As I sit and write now, the moment I wrote that, it was gon into the past. But I still consider it part of a more practical “now”. I am still in that same now for practical purposes unless I want to narrow it. When I wrote that last sentence it took an infinite number of nows. “Now” for us is the composite of experience we are able to capture in our awareness as a unique experience.
How short a now can you experience? Brain science can probabaly give us an answer. We cannot detect microseconds,