Visit <yoursite>/admin/settings.php?section=modsettingforum and and check 'Timed posts'. Create a forum post in a course and set it to expire yesterday. Create another forum post that is not starting until tomorrow. Create another forum post that sets no dates. Copy the URLs of each of these. Log in as student and visit the URLs. A message should display saying it is not available for the first two, and the third should be available.