I agree that the clock didn't start ticking until Adam and Eve sinned and Death actually became a process. I always wondered how long Adam was on earth before God created Eve.
Eternal is the nature which is characterized by timelessness, or rather the state wherein there time does not exist. It is considered to be timeless since it has no beginning nor end.
The concept originates from finite nature of this universe and all things seen and known that exist within it. It originates not from it presence or existence in this world but rather from the absence of anything within it, whether living substance or life process. So where is this illustrated in the Scriptures, try Ecclesiastics 3:1-8 wherein it is written:
1 To every thing there is a season, and a time to every purpose under the heaven:
2 A time to be born, and a time to die; a time to plant, and a time to pluck up that which is planted;
3 A time to kill, and a time to heal; a time to break down, and a time to build up;
4 A time to weep, and a time to laugh; a time to mourn, and a time to dance;
5 A time to cast away stones, and a time to gather stones together; a time to embrace, and a time to refrain from embracing;
6 A time to get, and a time to lose; a time to keep, and a time to cast away;
7 A time to rend, and a time to sew; a time to keep silence, and a time to speak;
8 A time to love, and a time to hate; a time of war, and a time of peace.
In the Bible, only the Creator is represented as the only 'eternal' one. Eternal does not mean absolute or infinite, it represents that which has the physical nature of being timeless, which is simply that it is has always existed and will always exist as signified within the first Chapter of Colossians, but in particular Col 1:17 wherein it is written "And He is before all things, and by Him all things consist."
So if Genesis 1:1 is considered to represent that in the beginning the Creator made the Universe, then the principle of the Eternal Nature would hold that since the universe and all things within it had a beginning then it therefore must have an end. In such I would aver to the Gospel in Matt 24:35, "Heaven and earth shall pass away, but my words shall not pass away."
Since the Eternal has neither a beginning nor an end, it can neither be created nor destroyed then anything which has a beginning or an end could not be Eternal, nor can become Eternal by the principle that it had in fact it began, or rather it did not always exist. I would be interested to hear anyone reconcile the belief that man was created eternal with the Scriptures of Genesis 2, since my opinion holds that Genesis 2 merely affirms John 3:16 and the resurrection of the spirit.