Formula help - calculating duration between dates in separate rows of data

Formula Queries, as Mark mentioned, would be quite good at this. However, if you have a lot of records (like he also said), the query will either not work or stall your app. If you run into that, Dons way works but you could also create a relationship to itself, and then use the Pipeline to relate 1 record to the next. This way you could use lookup fields instead. A different way, but I am not sure if it's any better?