New Post: Scheduled Publishing

How does the "Schedule Publishing" work? For example, I have a global datatype created with "Has Publishing" enabled. I create a new item and set published and unpublished dates. The item passes the requested publish date without publishing.

I am using a User Control to display the data on the page. Am I missing a step? How the the system update the datatypes on a schedule?

Thanks for any help you can provide. I know I am just missing something.
Rob