"... Even from everlasting to everlasting, You are God" (Psalm 90:2, NASB).
"Do you not know? Have you not heard? The Lord is the everlasting God..." (Isaiah 40:28, ESV).
"For thus says the high and lofty One
who inhabits eternity, whose name is Holy;
'I dwell in the high and holy place [outside the time/space continuum]" (Isaiah 57:15, RSV).
"[W]ho has saved us and called us to a holy life--not because of anything we have done but because of his own purpose and grace. This grace was given us in Christ Jesus before the beginning of time" (2 Tim. 1:9, NIV).
Most people don't know that time actually began! Before there was a cosmos, there was no time--it was not of the essence! "[G]od ... promised before time began" (Titus 1:2, HCSB, emphasis added).
To our disadvantage, we are confined to time to comprehend things because we fit into the four dimensions that it included; whereas God created time and is timeless; from everlasting to everlasting He is God, according to Psalm 90:1. Scientists have determined that if there were no space or matter, there would be no time. What we can infer is that they were all created ex nihilo or out of nothing by the command of God simultaneously, since time is merely a corollary of space and matter. If God created time, he cannot be defined in terms of it and is independent of it, and cannot be confined to it as we are.
God lives in more dimensions than we do and ones we cannot fathom. When we say God is timeless, it means He doesn't age nor change--He is immutable! Everything that begins to exist has a cause according to the ancient Greeks, and since God didn't begin to exist, He has no cause--He's not an effect, which means He owes His existence to no one or nothing. God is not an effect since He had no beginning! Therefore, He must be self-existent and is who He is. We don't know what God was doing before creation--some say He was creating hell for curious souls!
But Jesus is the "Alpha and Omega," who always is, always was, and always shall be; the One "who is, and who was, and who is to come, the Almighty" (cf. Rev. 1:8), having no beginning and no end. He is called the eternal "I Am" in Scripture and this is not an Is-ness or state of being, but an Is-ing, a living Being (the Is-ing One), that is independent of all other life.
What God did was create a place in our heart to long for eternity--He set eternity in our hearts according to Eccl. 3:11. When God is not defined in terms of time, it means He's timeless and will never grow old, become obsolete, nor become weary due to age. If God created time, then, He has the right to manipulate it according to His will and to step outside it and to see all of it in one big spectrum or perspective. To Him, a day is as a thousand years (He can accomplish as much as He wants in any given moment), and a thousand years is as a day (He can speed it up to His desires).
In summation, we can see how man cannot fathom God's limits nor understand eternity since the finite cannot grasp the infinite, as the Greeks said. We are limited no matter how we approach the equation of God. "Canst thou by searching find out God?" (Cf. Job 11:7). You cannot put Him into a box or equation! Jesus Himself pronounced: "... Before Abraham was, I Am [already existed as the eternal One]" (cf. John 8:58). God created the time/space continuum and is able to manipulate after His will. Soli Deo Gloria!