Originally Posted by potter

Yeah, I don't google is going to care because it's probably not checking your site more than once every 24 hours anyway. So whether you reload dynamic content every page refresh or once every 24 hours it'll likely not make a difference.

But, if you do want to continue to do what you're asking. Add a table/column/something to the database and just make it a simple unix timestamp. Everytime your page is loaded - prior to running the function that replaces the content - check whether or not time() is greater than 24 hours of the timestamp in the DB. If it isn't, you don't do anything - if it is you update the timestamp to the current time and replace the content.