The word aionios is generally translated as "eternal" and "everlasting" in modern translations. Hart translates the word differently, and his translation fits with his larger project articulated in his book That All Shall Be Saved: Heaven, Hell, and Universal Salvation. In fact, Hart has said that his translation of the New Testament functions as a companion piece for his argument in That All Shall Be Saved. And the primary supporting work his translation provides that argument has to do with the translation of aionios.
As Hart points out, there are only a small handful of passages in the Bible that, in modern translations, speak of "eternal punishment." One passage is Matthew 25.46, from the Parable of the Sheep and the Goats:
And these will go away into eternal punishment, but the righteous into eternal life.
And yet, as people familiar with these debates will know, there's a great deal of controversy about if "eternal" is the proper translation of aionios. Hart translates aionios as pertaining to an "age" or the "Age." For example, his translation of Matthew 25.46 reads as follows:
And these will go to the chastening of that Age, but the just to the life of that Age.
Regarding translating aionios as "age" or "Age" over "eternal," it is true that aionios can refer to a period of endless duration, but its far more common meaning was simply a "long span of time," or even just "a specific span of time," like a historical epoch, past or present.
Also, aionios can indicate qualitative distinctions (rather than quantitative durations), contrasting a mundane earthly existence with a blissful spiritual existence. This usage is common in the Gospel of John where we're told we are experiencing and participating in "aionios life" here and now, right in the midst of normal human life, the two "lives" existing side by side, a qualitative spiritual distinction here in the moment rather than a serial temporal sequence.
Finally, there are words in the Greek that do exactly mean "endless," but that word isn't aionios.
Hart also points out that the Jewish writers and the early church fathers closest to the New Testament texts didn't use aionios to mean "eternal," at least not primarily or exclusively.
All that to say, while, yes, aionios can mean "endless" it doesn't necessarily mean that, and odds are it doesn't mean "eternal" if you look at how the word is typically used.
So Hart chooses to translate aionios straightforwardly, reflecting its most common usage, as referring to a span of time, to an "age." And also, when context indicates this, an "age" with a qualitative distinction, "the Age" to come. You see that choice in effect in the translation of Matthew 25.46 above, a chastening or reward that awaits us in "that Age." Duration unspecified.
