If the site you're building is visited frequently, you can keep a timestamp (or datetime or whatever) in your database holding the next time the script has to run. Then include the script in your website (every page or just a selection them).
If the current time is equal or greater than the time in the database, run the script and set the time in the database to the value the script has to run next. In this way, the script will be executed by one of the visitors of the site, without them knowing.