Ask HN: Sorting items by rating & date?

I'm in the middle of writing a small app at the moment and I have come up against a roadblock with my understanding/terminology...

I'll use HN as my example:

When a new comment is posted, it temporarily out-ranks older posts that have a higher rating and then as it ages it works it's way down to it's final display position.

How is this achieved? I have doing some research to find out how it's done, but I'm obviously not looking in the right places as I have come up with diddly...

If anyone can explain this to me that would be great. Extra points if the sorting can be done in MySQL!