Gribbin (1990: 159):
Very few things in physics "care" which way time flows, and it is one of the fundamental puzzles of the universe we live in that there should be a definite "arrow of time," a distinction between the past and the future.
From the perspective of Systemic Functional Linguistic theory, time is a construal of experience as meaning. Halliday & Matthiessen (1999: 113):
Whatever the mode of occurrence of any figure, it will always unfold in time. This temporal unfolding is construed as an inherent property of the process itself, realised grammatically in tense and aspect;The location or extent (duration, frequency) of a process in time is construed as a circumstance of its unfolding.