Magento allows you to schedule custom tasks in an XML configuration, in a similar manner to the UNIX crontab style.

.....

        
            
                * * * 12 *

                mymodel/observer::runScheduledFunction
            
        
    
...

This example will run Ayasoftware_Mymodel_Model_Observer::runScheduledFunction method every December (* * * 12 *).

class Ayasoftware_Mymodel_Model_Observer extends Mage_Core_Model_Abstract {

public function runScheduledFunction ($observer) {
   Mage::log("nice to learn this ");
} 

} 

Update the Magento cron_schedule table, If for a reason X, you want to run this function at a specified time for example at 2010-05-21 17:45:00 , without having to change the configuration above.
This php code creates a pending cron job (created at $timecreated and Scheduled at $timescheduled):

$timecreated   = strftime("%Y-%m-%d %H:%M:%S",  mktime(date("H"), date("i"), date("s"), date("m"), date("d"), date("Y")));
$timescheduled = strftime("%Y-%m-%d %H:%M:%S", mktime(date("H"), date("i")+ 5, date("s"), date("m"), date("d"), date("Y")));
		try {
		   $coreResource = Mage::getSingleton ( 'core/resource' );
		    $write = $coreResource->getConnection ( 'core_write' );
		    $cron_schedule = $coreResource->getTableName ( 'cron_schedule' );
		    // Delete pending or completed crons 
		    $write->query("DELETE FROM  $cron_schedule WHERE  job_code =  'job_id'");
		    $sql = "INSERT INTO  $cron_schedule (
			        schedule_id ,
			        job_code ,
			        status ,
			        messages ,
			        created_at ,
			        scheduled_at ,
			        executed_at ,
			        finished_at
			        )
			       VALUES (
			        NULL ,  'job_id',  'pending',  NULL, '$timecreated' ,  '$timescheduled',    '0000-00-00 00:00:00',  '0000-00-00 00:00:00'
			        )";
		    $write->query($sql);
		
        } catch (Exception $e) {
            throw new Exception(Mage::helper('cron')->__('Unable to save This Cron Job'));
        }

If you want to use Magento core functions:

$timecreated   = strftime("%Y-%m-%d %H:%M:%S",  mktime(date("H"), date("i"), date("s"), date("m"), date("d"), date("Y")));
$timescheduled = strftime("%Y-%m-%d %H:%M:%S", mktime(date("H"), date("i")+ 5, date("s"), date("m"), date("d"), date("Y")));
$jobCode = 'job_id';
try {
	
	$schedule = Mage::getModel('cron/schedule');
	$schedule->setJobCode($jobCode)
        ->setCreatedAt($timecreated)
        ->setScheduledAt($timescheduled)
        ->setStatus(Mage_Cron_Model_Schedule::STATUS_PENDING)
        ->save();
} catch (Exception $e) {
         throw new Exception(Mage::helper('cron')->__('Unable to save Cron expression'));
}

Dont forget to setup a cron ( for the file {{base_dir}}/cron.php) that runs every 5 (for example) minutes to check whether there is any pending task.

Let me know if you have any question.