Why does the "Cask of Amontillado" take place fifty years in the past? Why is he telling it fifty years in the future? please explain fully!

I have discussed this in class, and my teacher had said that it was possible that Montresor was dying. He feels guilty, and needs to tell someone of his sinful deed. He is telling he reader exactly what he did. In the beginning of this story, Montresor states "You, who so well know the nature of my soul...". He is trying o get himself to believe that what he did was right. He wants to believe it, but he is finding it difficult to do so, so he is telling the reader what he did to get reassurance.