While we do not understand the spiritual world, we have glimpses into it. There is no reason the rules have to be the same as for our physical universe. And, there might even be a special set of rules for God.
God gave some hint with the burning bush in the Old Testament. He identified Himself as “I AM.” If someone asks you about a past event, say a sporting event you attended, you would answer in the past tense. This use of the present tense by God brings about the question of whether God uses time at all, or is simply always present external to physical laws.
Another hint comes from the reference to the end of days. Does this mean time will end, or simply that the sun will end? Does it refer to an actual day literally, or to time itself? We expect the sun to end, or at least cease to exist in its current form, and the endings may vaporize the Earth before the sun collapses into a cinder of a star. It is expected to grow in size to include the Earth’s orbit before the final collapse.
So, it appears God does not use time as we do. But, as long as we are alive we cannot understand what eternity is, since we cannot understand time as it pertains, or does not pertain, to God, and whether those rules also apply to other beings in the spirit world. Certainly some sequencing is involved in the spirit world, for after the revolt when Saint Michael cast the devil and his followers out there is no evidence they can simply go back before the event, or use a prior to the event existence. That singular event divides existence into before and after, and they do not intertwine even for spirits, except possibly God who is thought to be always in the present, a concept difficult for us to fully grasp. This implies sequencing in the spirit world with concepts such as before and after.
We can only conclude eternity means without end, but is it in the context of time as we understand it? That remains to be understood in the next world. And, an even harder question is what does without beginning mean, since time would not have existed in our current understanding before creation. Before matter came about would time even be able to exist, or would time have had a different form?