For most of us, shaped as we are by the cultural Enlightenment heritage, time is an a priori -- a given -- and has its own independent, objective reality. For those of us, time can be quantified in terms of past, present, and future. And eternity, then, becomes the infinite expansion of the future.
For Christians, though, there's the paradox of the Incarnation. The "end of time", τὰ ἔσχατα, is God's Kingdom. This Kingdom isn't simply some future goal, but is God's intervention into the world of space and time at particular moments -- most supremely in Jesus Christ. "In the fullness of time", Paul insists, "God sent his Son, born of a woman, born under the Law, in order to redeem those who were under the Law, so that we might receive adoption as children" (Galatians 4.4-7). So, whereas the Enlightenment conception is that time is an existential conception (rooted in human self-consciousness), the Christian conception is that time takes its meaning -- its "fulness" (πλήρωμα τοῦ χρόνου) -- from God's intervention in the world.
Time, as it's humanly experienced, falls into those three existential dimensions: Past, present, and future. The past is the time that has been. It is the time which has being as bygone, and is now the object of remembering and recollecting. The present is being in another mode: It is immediacy and presence in the existence we now know. By contrast, the future is neither bygone nor immediate, but is becoming and the object of expectation.
The time of Jesus -- particularly the resurrection and Easter -- designates a temporally unique epoch. The "forty days" between the resurrection and the ascension recorded by Acts becomes the hinge of revelation upon which turns the Christian experience of time. It is not merely something past; the resurrection is also the experience of the Kingdom in the world, and the hope which re-members the Church.
So the human time of Jesus' life bears this dual representational aspect of time for Christians. In part, God is represented in Jesus and therefore in the past; God himself is truly present to the world in the course of this temporal life. That's the point of Emmanuel.
But in Jesus, humanity is also remembered and recollected before God. In this one individual, the human experience of past, present and future collide into a new experience of time. Easter is the decisive experience of Kingdom "time". The Jesus who was present to his disciples was a life in time -- but this life redefines time by refusing to allow human experience to limit how we experience time. This is a life now fully present and the goal of the future -- his return which we now "remember".
This means that, for Christians, the time of the Risen One is unlike the limited human time.
In the Easter experience of the book of Acts and elsewhere in the New Testament, the first community encounters something other than existential time. They encounter Kingdom time -- in which the old creation passes into nothingness, and the new creation comes into being (2 Corinthians 5.17). The resurrection, therefore, for Christians, defines time by deconstructing human experience and narrating it new ways.
Human life is historical, but by its historicity is also transitory. There is nothing we can do outside Jesus Christ to change this. But in the resurrection, things change. Time changes. The Kingdom comes in its fulness, and the creation itself begins to groan for that redemption which is the New Creation (Romans 8). One of those creatures now groaning for redemption is time itself. And time finds that redemption in the resurrection and in Easter time.
"I am the Alpha and the Omega, the beginning and the End, who is and who was and who is to come" (Revelation 1.8). God's uncreated, Kingdom time becomes the new creation -- and therefore humanity's -- time "in Christ".
I think this is what Paul Tillich was hinting at in his notion of the "Eternal Now". The Lord God is lord over every other god -- including Kronos. This is why the earliest Christian confession -- κύριος
Ἰησοῦς -- is so significant. Any Christian theology or spiritual practice of time must begin there.