The definition of eternity given renders eternity a function of time. This definition requires that all the properties of eternity be limited to and explained by only the properties of time (and space) e.g. begin, end, reach, wait, start, now.
I believe the opposite to be true, that is time is a function of eternity. If so, then eternity possesses all the properties of time and other properties as well e. g. omnipresence
Since no one (able or willing to post in this thread) has experienced eternity, neither case can be proven.
Eternity belongs to God. There is no eternity outside His vision; there is what He has created, heavenly and in time and space.
We can reduce everything into some sort of summary, doing great disservice to what is, was and will be, and imagine that totality to comprise what it means to be eternal.
The universe is not eternal however, having had a beginning and an end.
God is eternal as the Source of time and thereby encompassing and within all time.
We can have a sense of the eternal right here and now, where we meet God in the rality of this moment which is given to us through His loving grace.
The past that was, likewise existed, connected to its Source.
Right here and now, eternity meets time.
Now has no boundaries; it cannot be said to last any amount of time and has no beginning or end.
Now just is.
And, it is our connection to God; this is where the encounter occurs, always has and always will.
Within our finite experience, with its limited attention span and capacity to stay focussed and not distracted by the world, it remains possible to know God and eternity.