From a Calvinist Systematic Theology:
The Divine decree is formed in eternity, but executed in time. There are sequences in the execution, but not in the formation of God’s eternal purpose. In his own mind and consciousness, God simultaneously because eternally decrees all that occurs in space and time; but the effects and results corresponding to the decree occur successively, not simultaneously. There were thirty-three years between the actual incarnation and the actual crucifixion, but not between the decree that the Logos should be incarnate and the decree that he should be crucified. In the Divine decree, Christ was simultaneously because eternally incarnate and crucified. “The Lamb was slain from the foundation of the world,”
William G. T. Shedd. Dogmatic Theology (Kindle Locations 5417-5422). Monergism Books. Kindle Edition.