There is nothing sufficiently evident to say time is objectively linear. Sure we can apply arbitrary measurements and ratios to it and call them seconds, minute hours etc and we can plot it on a chronological linear graph or culturally adopt a linear concept of it.

But this is artificial. The second isn't inherent or fundamental to the universe but it is no less used in the majority of our most fundamental physical formulas, equations and laws to describe it. The second is in truth a relative measure of us and the universe and if we lengthened or shortened the second the laws of physics would move proportionately with it.

What the second is however, is a cycle. The most observable objective and consistent measure we can make of time is frequency. The more consistent the distance between periods of a frequency the more accurate a measure it is of time. As ancient humans we understood the measure of time in this way, the cycle of the seasons, tides and moon, the zodiac and earth around the sun etc.

Nowadays, cycles can be a little more accurate and invariable in measure than the ones provided by nature but to do so one must reduce the spatiotemporal element. Go really small and really fast to get consistent frequencies that are less likely to be perturbed by external factors.

This is the atomic clock generations. Based on the incredibly energetic vibrations of electrons between energy levels as it decays and is the most modern and accurate way to define a second.

So we can measure time in frequency but is time itself a frequency? Couldn’t it change and never return to an original state? Yes it’s possible. But then how does one measure it? And if one cannot measure it is there any reason to believe it passes at all. Furthermore if we keep increasing the frequency of measure of time we get to a state so energetic it is pure Energy no mass therefore no electrons to measure the frequency of.

So how do we say energy has a speed? Speed is again a relative term to us and our cycles.