For this you're going to need to log a) the topic and b) 'digg's to the topic (lets call them votes for this).
When someone 'vote's a story the vote, user and date/time (or a timestamp) needs to be logged in the database. Then when you bring up the front page stories your algorithim needs to have a) the set amount of time - might need to be dynamic; b) whether or not the story reaches the amount of votes necessary.
I'm not sure of the code you'd need to find the number of votes spread over an amount of time however.
Yeah its kind of weird that your asking us this? Your the one that is suppose to determine your business logic, mate. For something like this... the programming part is easy... its determing what you actually want it to do... and how to determine it... those are all on your business logic rules :)