Ultimately, it seems to me, a fundamental aspect of the universe is real if, in principle at least, it can be measured either by comparing two quantities of it or observing its impact on a recording device. Time has always been measured indirectly through movement, be it of heavenly bodies, sand, melting wax, pendula, cogs, electric current or atomic oscillations. But no one to my knowledge has ever compared one second to another, or recorded the effect of varying quantities on a device, as we are limited to a presumption that movement over the same distance takes the same amount of time. Therefore, we can not even show that this artifice of time runs at one second per second. Notwithstanding 'highly accurate' atomic clocks changing state billions of times a second we are unable to demonstrate that one lot of oscillations took as long as the previous without first assuming the very quantity we are trying to measure. Distance, or rather displacement (an earlier post says change), is the actual observable effect with no tangible evidence to relegate it to mere proxy status for time. Is this a defeat not conceded in the move to redefine time as a distance travelled by light? In what sense then is time a dimension commensurate with space?