Stupid Cookie Question

Sorry for having to ask this because i'm sure its a really simple answer... I set a cookie with the following... <?php header("Location: index.php"); SetCookie("password","$password"); SetCookie("cwname","$cwname"); ?> User see's a form with username and password fields. It sets whatever he inputs to cookies. Now i'm trying to figure out how to delete them! I know you can delete it if you set a negative time but I never set a time to it in the first place. Thanks, Nate