Here is the way I look at God being eternal.
Moses went up a mountain, was it about 4000 years ago and saw God. When he came back down his face was shining (the glory of God). He also began to tell the people about a prophet like him that would one day come and that they must listen to him.
Elijah went up a mountain and to see God.
Jesus, James, John, and Peter went up a mountain about 2000 years ago, and there they saw Jesus shining brightly, and they saw Moses and Elijah on the mountain talking with Jesus.
When Moses went up the mountain, when Elijah went up the mountain, when Jesus and the three went up the mountain, all became aware of reality the way God sees reality; they were knowing with God’s knowing rather than their temporal way of knowing.
God knows (not “God knew”) Moses on the mountain. God knows Elijah on the mountain. God knows Jesus and the three on the mountain. And God knows his Son as Shining bright. It is apart from time. God knows that mountain and those six people there sharing his knowing plus our thinking about that moment, eternally.
When Moses went down, back away from knowing with God’s knowing, he came to his people and told the Israelites that another prophet like him was coming, “Listen to him”. As he about to leave the mountain, he had heard a voice from the cloud over his shoulder saying, “This is my beloved Son; listen to him”. It was not only Peter, James and John who heard that command, but also Moses and Elijah, all outside of time together in the knowing of, participating the knowing of, the presence of God.