At one time rather than another means that an originated thing begins at a definite time even though earlier and later times are conceivable.
If a possible entity comes into being today, its temporal beginning does not entail that God's will also began today. The date of the effect and the beginninglessness of the attribute are different considerations.
Confusing them yields two mistaken inferences: treating the creature as eternal or treating every new event as the origination of a new attribute in the divine essence. Neither follows from the doctrine of eternal will.