Adam Clarke on Psalms 90:2

Before the mountains were brought forth - The mountains and hills appear to have been everlasting; but as they were brought forth out of the womb of eternity, there was a time when they were not: but Thou hast been ab aeternitate a parte ante, ad aeternitatem a parte post; fram the eternity that is past, before time began; to the eternity that is after, when time shall have an end. This is the highest description of the eternity of God to which human language can reach.