Time can only be measured by periodicity. Time is only a concept and only a property of the cosmos in so far as it resides in the awareness memory of certain entities within it. The concept of time requires the perception of periodicity which in turn requires the phenomena of memory to confirm the regular recurrence of specific states, be they solar days, heartbeats, lunar cycles, or growing seasons. The concept of time is a consequence of memory and conversely without memory there is no concept of time. Time is the consequence of cyclic change and is thus a function of remembered periodic energy redistribution. If it could ever be contrived that periodicity was absent, then that condition would be essentially timeless. An entity in isolation without the capacity to perceive and remember heartbeats or rhythmic breathing would in effect be in a suspended animation of timelessness. For cosmologies to postulate durations for certain phases of the universe, there must have been periodicities of some sort during that phase in order to make comparisons. If there were no periodicities, there would have been no time.

The passage of time depends upon the regularity of the clocks being used to measure it. As technology reveals increasingly stable periodicities, so the reliability of established time-pieces can be shown to be suspect. To terrestrial life forms, the most influential periodicity is of course the solar day... whereby the sun routinely appears to cycle around the earth... but with the development of mechanical clocks it was revealed that it's duration varied by several minutes over the course of a year. The use of atomic oscillations to measure the passage of time was able in turn to demonstrate the inaccuracies of mechanical and electrical clocks. Whether it can be shown that there is a cosmic invariable oscillation against which all other periodicities can be compared... now, in the past, and in the future... will probably remain another of the uncertainties of the universe.

Residual evidence of cosmos configurations as it evolves and complexifies can be thought of as 'memories'. Individual awarenesses can interpret and extrapolate this evidence and thus enhance the ego-centric time concept of their allocated life-span. Thus for example, background radiation may be deemed evidence for a cosmic beginning, geological structures could be construed as evidence for terrestrial evolution and dinosaur fossils may or may not be deemed evidence for a cometary impact in the distant past. Old buildings and museum artefacts can engender a sense of historical order and documentary evidence can selectively enhance the model of one's family tree.

Time is not a tangible, physical, independent entity or property. Suppositions that it could be a travel medium are not examples of inspired thinking but of delusional nonsense. Proposing for example, that the slowing of an entities intrinsic periodicities is some sort of time travel... by freezing it, or racing it there and back at the speed of light... is a linguistic deception. Inhibiting the natural growth of something is called 'developmental retardation' not time-travel. Entities, beings, awarenesses, and things in general are intrinsically connected to their time and place by an uncountably large number of links and chaotic connections, such as family ties, cultural nurturing, travel adventures, the composition of foods and water, the accumulation of memories, and interactions with symbiotic and pathogenetic flora and fauna. The absence of all existential links to an entity is evidence that it doesn't exist... be it past, present or future.

Time cannot be 'reversed'. Any form of 'time reversal' would require most if not all of the behaviours of the universe to be reversed... gravity in particular. The newtonian apple which fell to earth a moment ago would have to be repulsed and rise up if time was to be reversed. But gravitational orbits and periodicities are only possible with an attractive gravitational force. The attractive force of gravity controls the periodicities of orbits, the evolution of stars, the pendulum of a clock, the water cycle and the geological sedimentation of strata. A universe with repulsive gravity could not exist. The universe evolves as it does because of its array of intrinsic behaviours, and time is a consequence of the specific periodic nature of those behaviours.