Upon the floor of light, and time, Unmurmuring […] We rest; (9-11)
You think you're on to something solid, but then you have to do a double take. Did he say "floor of […] time"? Is this like the ground floor of something bigger? We know the lovers are lying on the ground, but this ground is made of… light and time. How can that be? It's as if they're at the beginning of something still and silent, awaiting a glimpse of something bigger.
Ages to our construction went, Dim architecture, hour by hour: (13-15)
Get ready to have your mind blown. This may be just an hour on an afternoon, but just think about all the time (and "construction") that went into building up to this—the whole of human and natural history. Every now is composed of zillions of hours and minutes and seconds, in a process of incremental evolution, just as the shores are made of grains of sand.
If hope is hopeless, then fearless fear, And history is thus undone (27-28)
Are things really that hopeless? Is the speaker just throwing in the towel? We don't think the speaker is telling us that all that happened in the past didn't really happen, but without hope or fear, the story unravels, doesn't it? Without these emotional forces, there's no human drama.
We live in time so little time And we learn all so painfully, That we may spare this hour's term To practice for eternity. (37-40)
Say it enough, and the word "time" begins to lose meaning, or to mean nearly everything. Our time is short, that's for sure. Time is almost a synonym for life here; its elapse is our own span. Should you fill your time with activity, or, like these lovers, spare one hour just lying there doing nothing, saying nothing, and getting ready for the big… nothing, the timelessness that is eternity? Decisions, decisions…