We understand eternity by removing things from time, but time has three levels of transcendence which allow us to flesh out the account of eternity by extrapolating the data points that get plotted by progressive removals.
1.) Time in the natural world. We can draw attention to four features:
a.) Distinct events. This event is not that one.
b.) Irreversibility of events. Minimally, earlier events always happen before later ones and not vice-versa.
c.) Continuity of events. To get from one moment to another one has to go through indefinite other moments.
d.) Exclusivity of perfections. Perfections peculiar to one state can’t be enjoyed at another. What belongs to the earlier as earlier can’t be found in the later as later.
2.) Time in animals with memory.
The distinction of events and their irreversibility remains, but the animal’s memory and anticipation allow it to experience the past as past at later times, and to “leap over” all intermittent times. The animal, however, has this knowledge and acts by it without his knowledge itself being an object (animals see physical things, but not physics; living things, but not biology) and in this sense the exclusive perfections of time (1) are overcome only unconsciously. .
So (a) and (b) remain, but (c) drops out and (d) is overcome, but only subconsciously.
3.) Time in intelligences.
Both human beings and the angels experience the distinction of events but some events (like premises in an argument) have a before and after without having a temporal before and after, and they are known at once without this being a temporal at once. It is meaningless to talk about the time interval of modus tollens or the the general gravitation equation or a moment in which either makes its inference. Intelligences are also conscious of their own consciousness and so compare their acts to the world, causing it to be experienced as true or false and in other ways that transcend time (1) and (2). Nevertheless, this overcoming of the exclusivity of time is merely intentional or mental and not real.*
So (a) remains, (b) and (c) drop out and (d) is overcome consciously. Nevertheless, there is no overcoming of (d) in the real order but only the highest possible intentional order.
At this level the possession of perfections transcends limitation to the cognitive order and becomes real. Past events thus have all the perfection of fixity and certitude, but lack the imperfection of frozenness or unreality. Future events maintain all of the perfection of being anticipated and containing indefinite possibility while losing the imperfection of being impenetrable to intelligence. The present maintains its reality and its power to surprise while losing its fleetingness and fixity of the action occurring within it. Any being who lives here not only compares his belief to the world, but sees his world as containing all the reality that is ontologically divided at the lower levels. In a way similar to time (3) this containment of all times occurs at once, but not in the at once of time. Asking when eternal beings exists makes only as much sense as asking when abstractions or laws exist.
So (a) remains as a notional distinction and all the rest drop away entirely.
*That said, it is not obviously and straightforwardly true that there is a meaningful answer to when an intelligence (like your own, say) exists. Intelligence exists when its act does, and its act is an abstraction. But when does an abstraction exist? How long has it done so?