You could utilize cron to do something like alter the db to put the site into offline mode (via osticket db).
Or you use cron to shut down the site, fire up another site that serves a page that displays the hours the site is open.
Or you could write a script that checks the time, and if it is in the time frame passes the user along to the index.php, other wise display a custom hours.html
If you are running Apache you could make a custom .htaccess file to limit access based on time.
