Jesus was slain from the foundation of the world.
Time and space are limitations that we are not naturally able to step out from under the control of. Not so with Jesus. Before Abraham was, He still is.
All the intentions Yahweh has had toward humanity for blessing and Life were accomplished outside of time, and poured into the earth through Jesus while He was on the cross. All His Father had intended and promised to His faithful in the earth became “Yes” and “Amen” through Jesus in a moment on the timeline, and Jesus declared them “finished” when they were manifested in time.