Updating node publication date from hook_nodeapi doesn't work

Hello,

I'm trying to update a specific node type's date when the user create / update it. The purpose of this modification is to set the node's date time to a specific CCK field. Here is my code:

function cleandate_nodeapi(&$node, $op, $a3 = NULL, $a4 = NULL){
	if($node->type == 'image_of_the_day')
	{
		drupal_set_message('On Node ' . $op . ' - Date: ' . $node->date);
		switch ($op) {
			case 'validate':
				$node->date = $node->field_publication_date[0]['value'];
				drupal_set_message('After ' . $node->date);
				break;
		}
	}
}

I do have the log messages but the problem is that the $node->date is never saved no mater what I try. Am I missing something ?

Thanks in advance.