Scrooge notices the aging of the Ghost of Christmas Present as midnight approaches. What does the Ghost explain that is aging is due to?

The Ghost of Christmas Present, having already aged, reveals that he will only exist on Earth for a single year's Christmas holiday. (As the nature of the present is to only exist in the now, this is why this ghost can only exist for one Christmas, and why he has 1842 brothers. Note the year that Charles Dickens' story was published. This would be the 1843rd Ghost of Christmas Present.) He finally disappears at the stroke of midnight on Twelfth Night, and leaves Scrooge to face the Ghost of Christmas Yet to Come, as it approaches "like a mist along the ground".