you can vreate scheduer jobs, like Forum module does for sending emails. Be aware that this requires the web site to be active, an unloaded (recycled) IIS application cannot run any jobs. If execution on precise time is necessary, you might need a job in Windows scheduler or SQL server scheduler.

"Active" means that the application (IIS app Pool) is not unloaded or recycled - default configuration unloads applications after 20 minutes of inactivity. 

Besides, it depends, how the scheduler is triggered - this can be specified in Host Settings:

• "Request mode" starts the scheduler everytime a page is called
• "Timer mode" starts a background thread, which is running as long as the app is active. Disadvantage: the scheduler is missing portal context.

If the scheduler is not active at the designated time, the job will be executed when the app is restarted, you may specify, if missed multiple scheduled events shall be executed at once.