Time present and time past Are both perhaps present in time future, And time future contained in time past. (1-3)
Right off the bat, the speaker wants you to understand and accept the fact that the past, present, and future are not three different times. They're all the same, because time is a single seamless fabric. It's not clear at this point why this is so important, but as you read on, you realize that the speaker thinks our normal conception of time makes us sad by taking us away from the present moment.
I can only say, there we have been: but I cannot say where. And I cannot say, how long, for that is to place it in time. (70-71)
The speaker wants to show us the experience of spiritual enlightenment, but he has a really tough time doing this in words. This is because our language doesn't really provide us with the tools we need to talk about an experience that's basically beyond words. The speaker can only say that the spiritual experience is "there," and point toward it, though he can't say much more than this. He also can't say how long we've been in this place, because that will just put it into our normal concept of time, which is not how a spiritual experience works. It's something that takes place in a sort of unending present moment, where clocks aren't welcome.
Yet the enchainment of past and future Woven in the weakness of the changing body Protects mankind from heaven and damnation Which flesh cannot endure. (81-84)
The reason we worry so much about the past and future is because we know our bodies are always getting older, and this concerns us because it means we're going to die someday. It's because our time on Earth is limited that we obsess over the past and future. But if we're going to start living in a constant spiritual present moment, we're going to have to get over this fear of death and aging.
As in their living in the living seasons The time of the seasons and the constellations The time of milking and the time of harvest The time of the coupling of man and woman. (220-223)
People of the past had a healthier relationship to time than modern people. They were connected to the land because they were mostly farmers, and this connected them to the changing of the seasons, which happened in a constant circle. Compared to the dead emptiness of clock time, the seasons are "living," and throughout this passage, you can see how a connection to the natural world is connected to a more natural type of time, which is controlled by when the cows need milking, not some lame alarm on your bedside clock.
The tolling bell Measures time not our time, rung by the unhurried Ground swell, a time Older than the time of chronometers. (426-429)
The image of a tolling bell pops up throughout the later stages of "Four Quartets," always reminding us of the reality that death will one day come for all of us. It also signals a type of time that is way older than the clock-time ("the time of chronometers") we tend to think of in the modern world. There's nothing natural about the seconds and hours that clocks tick off. These are totally arbitrary segments of time that humans just made up. The changing of the seasons, though, is a natural way for the Earth to track the passing of time, and we need to get back to relating to the Earth in a more natural way if we're going to be happy.
We cannot think of a time that is oceanless Or of an ocean not littered with wastage Or of a future that is not liable Like the past, to have no destination. (460-463)
There's just no getting around it. You can't think of time without seeing a massive void that's littered with the waste and crumbling ruins of human history. Further, you can't think of a future or past that's actually heading somewhere in a progressive way. The past and the future don't have any destination; this is just something that we humans like to believe because we think it makes life more meaningful. The truth is, though, that life can only be meaningful if you let go of this need for progress and just start living for the present moment.