Why did Plotinus believe that the universe had no beginning in time?

Plotinus maintained that the material universe has no beginning in time because he viewed the entire process of generation as an eternal, timeless act.1

In his philosophy, the One (or the Good) is the transcendent source from which all reality proceeds through a continuous, necessary, and spontaneous process of emanation. Within this hierarchy, Nous (Intellect) and Soul are eternal. Because time is defined as the life of the Soul as it is active in the physical world, and because the Soul is eternal, Plotinus concluded that there never was a time when the material universe did not exist. Thus, the universe is a living, organic whole that reflects the eternal unity of the world of forms.1