The story of the book is of a couple who, through a series of life-circumstances, come to find themselves at Oxford and interacting with CS Lewis. This began first by letter as Lewis recommended books for them to read, then would discuss what they read. Their exchanges deal with fundamental beliefs: likelihood of God, likelihood of redemption, and the need for salvation. The portion of the letter below is from Lewis back to them and I am intrigued by the way he frames the idea of time.
It is quite clear from what you say that you have conscious wishes on both sides. And now, another point about wishes. A wish may lead to false beliefs, granted. But what does the existence of the wish suggest? At one time I was much impressed by Arnold's line "Nor does the being hungry prove that we have bread." But surely, though it doesn't prove that one particular man will get food, it does prove that there is such a thing as food! i.e. if we were a species that didn't normally eat, weren't designed to eat, would we feel hungry? You say the materialist universe is "ugly." I wonder how you discovered that! If you are really a product of a materialistic universe, how is it you don't feel at home there? Do fish complain of the sea for being wet? Or if they did, would that fact itself not strongly suggest that they had not always been or would not always be, purely aquatic creatures? Notice how we are perpetually surprised at Time ("How time flies! Fancy John being grown--up and married! I can hardly believe it!") In heaven's name, why? Unless, indeed there is something in us which is not temporal.
I like this because I have felt it, but never articulated it. It's kind of the ontological argument for not belonging here. We don't feel time moving on as it moves on, but only when we look back and realize how much has gone by. I think most of us recognize time moving by with regret, understanding that we will never get it back. Heaven, being timeless, will redefine eternity for us. We think of eternity as never beginning and never ending, but even as we define it, we do so using terms of time. We are prone to define it as what it is not.
