Aristotle: So What Kind of Good Lasts a Day Anyway?

This is another question regarding "Nicomachean Ethics" that was raised due to a frustrating lack of specificity in the text.  Or perhaps I'm just not seeing it plainly enough.

So Aristotle seems to hold the belief that the time of existence of an Ideal subject (goodness, for instance) does not affect the essence of that subject.  He says, “Nor yet will the Ideal Good be any more good because it is eternal, seeing that a white thing that lasts a long time in no whiter that one that lasts a day” (21).  Well, that's all well and good, but what doesn't help the understanding of this nuance in Aristotle's argument is that he never bothers to give examples of, and so help to define, just what the difference is between the one-a-day goodness and the goodness that supposedly lasts forever.  In comparison to his previously mentioned statement on goodness, he says that "seeing that one and the same definition of man applies both to 'the Ideal man' and to 'man', for in so far as both are men, there will be no difference between them" (19-21).  What does that mean, and how does he think that it provides a satisfactory comparison for his similar statement on goodness, which follows right after that in the text?  Whether or not there is no difference between the "Ideal man" and a regular man does not matter when compared to short and long-lived goodness.  Goodness can be eternal, according to Aristotle.  Man is not immortal however, so the comparison doesn't really make sense, and it only confuses the potential definition of eternal vs evanescent goodness.  Also, are we not as human beings affected by the knowledge of our mortality?  Would that not affect our philosophy?  Is he just ignoring that eventual cease of clock hands in his comparison of man to eternal goodness?  Whatever the case, the fact that he seems to think that comparing the difference of the Ideal and the regular in man to the same in goodness will so help define that goodness is wrong.  It just confused me more. 

It's true that later in the text Aristotle clarifies his definition of the Good.  First, he defines goodness as having two meanings "(1) things good in themselves and (2) things good as a means to these...  But what sort of things is one to class as good in themselves?  Are they not those things which are sought after even without any accessory advantage, such as wisdom, sight, and certain pleasures and honors?" (21-23).  This does not cut it.  Yes, he went further into vaguely defining goodness in general, but my question still remains.

If there is no difference between the "ideal" or eternal good and the good that only lasts a day and he is trying to prove that to us, what is the difference between the two that would have made his listeners and readers doubt him anyway?  Against what difference is he arguing?  What is it?  What examples might he have given us?