Isaiah writes that God "inhabits eternity," that is, He dwells in perpetuity or lives eternally, continually. Moses puts it a different way in Psalm 90:2, "Even from everlasting to everlasting [or age to age], You are God." However, the way Isaiah constructed the phrase, "inhabits eternity" can mean that God moves freely in time; any period of time is accessible to Him. He made it and has power over it. Whether this was Isaiah's actual intent is unknown.
Understanding this is made more difficult because Hebrew has no general word for "time." Ad, the word used in Isaiah 57:15, simply means "duration, perpetual, continuity." This is similar to the idea behind the name Yahweh, translated "LORD," which means "He who is." This corresponds to "'I am the Alpha and the Omega, the Beginning and the End,' says the Lord, 'who is and who was and who is to come, the Almighty'" (Revelation 1:8). God is, has always been and always will be, no matter how men perceive time.
Another of God's names, "I AM" (Exodus 3:14) also implies that men cannot truly understand His relationship to time. Robert Young, author of Young's Analytical Concordance, writes of this word, hayah, "A name indicating rather the unsearchableness of God than his mere existence, as commonly supposed" (p. 506, his emphasis). As Paul points out in Romans 11:33, "Oh, the depth of the riches both of the wisdom and knowledge of God! How unsearchable are His judgments and His ways past finding out!"
God's eternity allows Him to work out His plan over the whole expanse of time. From the most remote past, He has planned, created, and fulfilled each step of His purpose to bring about His ultimate goal, the birth of sons and daughters into His Family (II Corinthians 6:18). God Himself explains how this works:
Remember the former things of old, for I am God, and there is no other; I am God, and there is none like Me, declaring the end from the beginning, and from ancient times things that are not yet done, saying, "My counsel shall stand, and I will do all My pleasure," . . . Indeed I have spoken it; I will also bring it to pass. I have purposed it; I will also do it. (Isaiah 46:9-11)
Because of God's endless life and His power over events and lives of men, He can prophesy a thing to occur in ancient times and bring it to pass today. Only a Being unconstrained by time could carry out such a long-term feat.