The clock is meant to represent the "timelessness" of Eternity. Spinoza points out in several places that Eternity (as he uses the term) has nothing to do with time or duration. Although our ordinary life seems to us to be a sequence of events that follow each other this appearance, Spinoza shows, is really a confused presentment of our own imagination with the association of impressions on our body arising from "the fortuitous play of circumstance" through our senses. [It has been said] that "Now is the only real moment" though I believe [it] is meant by this more than just the "now" that seems to sit between past and future and to be continually moving. "Now" in this sense is the "Eternal Now" and this is very hard to find because our imagination is filled with fragmented ideas that cloud our "inner vision" and turn Eternity into time. Spinoza says:

============ E5: PROP. 23, Note:
"...Thus, although we do not remember that we existed before the body, yet we feel that our mind, in so far as it involves the essence of the body, under the form of eternity, is eternal, and that thus, its existence cannot be defined in terms of time, or explained through duration. Thus our mind can only be said to endure, and its existence can only be defined by a fixed time, in so far as it involves the actual existence of the body. Thus far only has it the power of determining the existence of things by time, and conceiving them under the category of duration."
============

He says elsewhere:

============ E5: PROP. 34 Corollary, Note:
"If we look to men's general opinion, we shall see that they are indeed conscious of the eternity of their mind, but that they confuse eternity with duration, and ascribe it to the imagination or the memory which they believe to remain after death."
============

    In the clock both the confusion of Eternity with Time and the idea that Eternity is "beyond" or "outside" of Time are meant to be represented. You see the hand moving from what is usually taken to be moment to moment but then perhaps you notice that all "moments" are Now. The imagination can make nothing out of this but a sequence of separate moments while to the Understanding it is quite another thing. Understanding, which term has a very specific meaning in Spinoza, is not the imagination, memory, or words but rather is the very act of affirmation which arises from our Essential Nature as a thinking thing. Our Essential Nature is not some independent being, as we usually imagine it, but rather is a particular expression of the Infinite and Eternal One, without which nothing has being and beyond which there can be no being.

    Years ago I took an hour glass and removed the sand. You never have to turn it over, it always represents "Now". Of course "always" usually means an infinite amount of time and we can hardly escape that illusion while the body "endures". Even this idea of the body "enduring" for some period of "time" is really a confused idea in our own complex bodily sense associations that is our imagination and memory. Even while this confusion continues, Spinoza points out, we can learn to "see" beyond it the Eternal and Infinite One.

    These ideas are very simple but it is a paradox of true Understanding that Simple, Clear Ideas are often the hardest to grasp while the complex but confused ideas of our own imagination run rampant and present to us the continually changing illusion of life. Real Life is Eternal, Infinite, and Unchanging. Spinoza points out:

============ E2: PROP. 47, Note:
"...Men have not so clear a knowledge of God as they have of general notions, because they are unable to imagine God as they do bodies, and also because they have associated the name God with images of things that they are in the habit of seeing, as indeed they can hardly avoid doing, being, as they are, men, and continually affected by external bodies."
============