On The Meaning of Now
Polymath Yoav Netzer with a profound and short meditation on The Meaning of "Now"
The meaning of 'now' may be physiological and cognitive e.g.
1. The physiological phase-lags of the physiological feedback (aka closed-loop) control systems like heart-pulse, breathing, movement, temperature.
Thus under pressure, when the heart pulse rate increases, the 'now' shortens.
2. Cognitive response times up to the OODA loop (sketchy, was drown by a pilot non-engineer, yet gives an idea of a wider response context).
Essentially, the individual 'now' may reflect, aka be proportional to, a number of one's own pulse and response pulse-lengths (wavelengths).
Thus the 'now' is in the human language rather not in the sheer physics.
The difference between 'now' and 'later' is an accounting one, thus a language which grammar comprises a distinct genitive (ownership) case is also likely to make a clear grammatical distinction between the present and future tenses.
Stingy, thus also Time-sensitive cultures, like English and German, have indeed several tenses to describe various lengths of the present and the future.