How to add value based on 24 hours passing on server?

Say I have a number, like 10,000. I want to add a value, say 6, every 24 hours. The site is basically keeping a running count of a product & they make 6 every day. I really have no idea how to tackle this as far as adding a value of 6 every 24 server hours.
Thanks for any help.

This is best done on the server. JavaScript starts over each time the page loads and so would only be able to do the update if someone were to keep the page open for over 24 hours.