I'm not sure how HN does it specifically. In a version of our app, we had scoring which was initially modeled after Reddit's algorithm (http://amix.dk/blog/post/19588). A server task updated the scores for every item every hour. We could have limited how far back we went, but had no need in our case given that the amount we indexed was within reason. We'd add or subtract from the score based on a set of rules that considered a number of factors including posted date, number of votes and number of comments. We had an admin tool for assigning more or less weight to the various factors.