R. A. Muller's book Now (Norton, 2016) has made me think about the concept of "now." For me the contents of short term memory define "now" and these contents can vary somewhat from one person to the next (as they do in standard relativity theory).
The "past" is then the contents of long term memory.
The "future" is any predictions. (Includes results of chaining, extrapolations, etc.)