That God is eternal does not mean he is not also in time. There is no contradiction between the two modes of being; if there were, then there would no way to have temporality in the first place; for, since eternity is prior to time, time is happening in eternity, and is fully limited by and conformed thereto.
So, God responds to us in time, just as we respond to each other. His response is happening in time and in eternity – in time, which is an aspect of eternity. So Jesus is in time as we are, but he is also consciously eternal. The Incarnation happened before all worlds because all worlds happened before all worlds. The happening of worlds is a procedure of eternity.