[Adrian Schneider]

PHP Code:

$db->query_write("
    INSERT INTO table_name
        (user, message, date)
    VALUES (
        '" . $db->escape_string($username) . "',
        '" . $db->escape_string($message) . "',
        '" . $db->escape_string($date) . "'
    )
"); 


You probably won't have to escape $date though (you should be saving it as a Unix timestamp).

Edit: instead of using raw $_POST values, use vBulletin's cleaning functions (read more here: https://vborg.vbsupport.ru/showthread.php?t=98047).