So maybe you're on vacation, you're in Athens for the first time. During the day you're experiencing a bunch of new events and new sights and sounds, and as it's going by—that is prospectively it seems to be flying by. In retrospect however, maybe the next day or you're back from vacation looking back upon that, it seems to be a long day. So retrospectively it seems that it was an extended period of time. And this is something that was pointed out as far back as William James in his Principles of Neuroscience over a hundred years ago. And the point is that retrospectively we're not so much telling time but we're rebuilding or estimating how much time has elapsed based on the number of experiences we have in memory. So retrospectively you're more estimating how much time has elapsed, if there was a period full of new memories then you're left with the impression that it was a long period of time retrospectively. But prospectively as it was taking place you were paying attention to some sort of internal clock in our brain, which you were looking at or querying, that was telling you that not much time has elapsed, because you weren't paying much attention to time. So on the other hand when you're very bored or in an anxious state people experience time as dragging or going slow (again, prospectively).