In response to an argument that an eternal thing is something that remains throughout all time, and is therefore is temporal, St. Thomas says:
Taken per se, time is the measure of the first motion, and so temporal existence is not measured by time except as subject to the variation of the motion of the sphere (caeli). Thus Averroes says that we sense time so far as we sense ourselves to exist within the variable existence of the motion of the sphere. It follows that all things that are caused by the motion of the sphere, the first measure of which is time, are measured by time, and and everyone who senses any sort of variation that follows the motion of the sphere senses time even if he doesn’t see the actual motion of the sphere.
[T]empus per se est mensura motus primi; unde esse rerum temporalium non mensuratur tempore nisi prout subjacet variationi ex motu caeli. Unde dicit Commentator, quod sentimus tempus, secundum quod percipimus nos esse in esse variabili ex motu caeli. Et inde est quod omnia quae ordinantur ad motum caeli sicut ad causam, cujus primo mensura est tempus, mensurantur tempore; et quicumque sentit quamcumque variabilitatem quae consequitur ex motu caeli, sentit tempus, quamvis non videat ipsum motum caeli.
Sup Sen I. q. 19 a. 2
To speak of time per se means to speak of it as it is understood through the concrete account of a physical theory; and the very perception of time is the perception of what we see through a theoretical account.
My suspicion is that time is particularly difficult to deal with by common notions and their subsequent dialectical development. This is borne out even in Aristotle’s Physics – for even though he could say a great deal about motion from common principles, when he speaks of time he very quickly has to bring in the outermost sphere (though it comes up even more quickly in his account of place).
I’m interested in this primarily as a metaphysician, and with an eye to speaking of eternity, but this seems to be a cue that my idea of eternity should make more reference to light cones, entropy (and therefore information), and the diffusion of causes throughout nature (as opposed to the higher causes being in a different place from the lower ones.)
The difficulty in doing this, however, is that eternity is not a measure of being that can be derived from the sort of account of the world in which entropy makes sense. Eternity is not constructable by the human effort, nor do we have any relevant units in which we might give eternity an algebraic expression.