Before the formation of the world, when there was no sun dividing day from night; there was only the aeon that is coextensive with the things that are eternal like some temporal movement or interval. In this sense there is a single aeon, in according with which God is said to be aoenial, but also pre-aeonial, for he himself also made this aeon. (John of Damascus, On the Orthodox Faith, 97-98)
What is before creation? It seems that, in the history of Christian thought, the idea of "time" before creation was explored in the concept of the "aeon" (αἰων), a Greek term normally translated as "age." John of Damascus explored this concept of the "aoen" as God's first "creation" (in Section 15) before the actual creation of the universe in Section 16, which were followed by discussions of the "invisible world" (Sections 17 and 18), the visible creation (Section 19), then the various frames and elements of the material world as understood at that time (Sections 20 to 24b). Therefore, the idea of "time" before creation, even an eternal "time" before creation, was held at least by John of Damascus. The "aeon" is made by God, yet it has a derivative "eternity" not linked to the essence of the eternal God.
It is this manner of talking that is interesting for our modern times, if only for the fact that much of the polemics coming from modern classical theists imply that anything that is eternal must be linked with God's essence. Yet, as we can see, this does not seem to be the view held to by John of Damascus, who is considered one of the later church fathers. We can speak of an "eternity" that is "aeonic" in nature, and this is not a heretical or heterodox position but a perfectly orthodox one.