The Demiurge's model — the eternal Living Creature after which the cosmos was made — exists in eternity. Eternity, for Plato, is not endless time but timelessness: the unchanging self-identity of the Forms, which neither were, nor will be, but simply are. To bestow this in its fullness on a creature would be impossible, for the cosmos is made partly of matter, which changes. So the Demiurge did the next best thing: he made an image of eternity that moves.
Time was created together with the heavens: there were no days, nights, months, or years before the cosmic order was established. The regular motions of the celestial bodies are what time is — not mere instruments for measuring a pre-existing flow, but the very substance of time itself. This has a striking consequence: to speak of time before the creation is to speak of nothing, since time just is the ordered motion of the cosmos.
Plato goes further. Even our grammatical tenses are misleading when applied to eternal things. We say of the eternal Forms that they 'were' or 'will be' — but this is wrong. Only 'is' truly applies to them, because 'was' and 'will be' describe motion through time, and the eternal does not move. Time is the region of becoming; eternity the region of being; and the mistake of importing the grammar of one into the discourse of the other is a philosophical error Plato wants to correct.
What makes Plato's account memorable is its tone. Time is not a fall from eternity, not a corruption or shadow of the real. It is a gift: the best approximation of eternity that the material world can sustain, crafted deliberately to honour its original. The planets revolve in ratios that reflect the mathematical structure of the world-soul; the turning of the heavens is the closest a mortal cosmos can come to the stillness of the eternal. Time, on this account, is not the enemy of permanence — it is permanence's most faithful image.
The account of time appears in Chapter 2 of the Timaeus. It became enormously influential: Augustine's treatment of time in the Confessions is partly a response to it, and Plotinus's account of time as the life of the soul restlessly imitating eternity develops the same core idea.
