Below is the clearest explanation I have found so far of the 'eternal now' concept...
This quote from Paul Tillich, comes from his book The Eternal Now, at the end of chapter 11, which deals specifically with the concept of time, and an "eternal now". Here it is:
The mystery of the future and the mystery of the past are united in the mystery of the present. Our time, the time we have, is the time in which we have "presence." But how can we have "presence"? Is not the present moment gone when we think of it? Is not the present the ever-moving boundary line between past and future? But a moving boundary is not a place to stand upon. If nothing were given to us except the "no more" of the past and the "not yet" of the future, we would not have anything. We could not speak of the time that is our time; we would not have "presence."
The mystery is that we have a present; and even more, that we have our future also because we anticipate it in ‘the present; and that we have our past also, because we remember it in the present. In the present our future and our past are ours. But there is no "present" if we think of the never-ending flux of time. The riddle of the present is the deepest of all the riddles of time. Again, there is no answer except from that which comprises all time and lies beyond it -- the eternal. Whenever we say "now" or "today," we stop the flux of time for us. We accept the present and do not care that it is gone in the moment that we accept it. We live in it and it is renewed for us in every new present." This is possible because every moment of time reaches into the eternal. It is the eternal that stops the flux of time for us. It is the eternal "now" which provides for us a temporal "now." We live so long as "it is still today" -- in the words of the letter to the Hebrews. Not everybody, and nobody all the time, is aware of this "eternal now" in the temporal "now." But sometimes it breaks powerfully into our consciousness and gives us the certainty of the eternal, of a dimension of time which cuts into time and gives us our time.
People who are never aware of this dimension lose the possibility of resting in the present. As the letter to the Hebrews describes it, they never enter into the divine rest. They are held by the past and cannot separate themselves from it, or they escape towards the future, unable to rest in the present. They have not entered the eternal rest which stops the flux of time and gives us the blessing of the present. Perhaps this is the most conspicuous characteristic of our period, especially in the western world and particularly in this country. It lacks the courage to accept "presence" because it has lost the dimension of the eternal.
"I am the beginning and the end." This is said to us who live in the bondage of time, who have to face the end, who cannot escape the past, who need a present to stand upon. Each of the modes of time has its peculiar mystery, each of them carries its peculiar anxiety. Each of them drives us to an ultimate question. There is one answer to these questions -- the eternal. There is one power that surpasses the all-consuming power of time -- the eternal: He Who was and is and is to come, the beginning and the end. He gives us forgiveness for what has passed. He gives us courage for what is to come. He gives us rest in His eternal Presence.
I believe that so-called ‘time’ and ‘space’, which are really one, are no more than media in which all things exist. Life is movement, ceaseless movement and life itself is timeless and space-less. That much is clear. Another thing is clear. Everything is contained within ‘the Now.’ All duration or time is total and complete in 'the Now'.
There is an ‘eternal’ quality about the Now. It is forever new.