Truly do not know much about mathematics and know less regarding Mr. Nash. Any mathematician looking at our world and universe does have my respect though because of the intrinsic nature of the beast called ‘time’. Many mathematicians consider ‘time’ to be irrelevant but how can one be positive ‘time’ or the ‘measure of time’ is irrelevant?
I believe positive Tim and negative time is in relation to your current position.
Einstein's theory states that there is no such thing as absolute time. Time ticks at different rates for observers in relative motion. In short, a moving clock ticks out time more slowly than your own stationary clock. All because of the fact that the speed of light is the SAME for ALL observers, regardless of their state of relative motion.