How do you delete a cookie

I can create a cookie in a function say.

function make_cookie($salt, $userid)
{
$C = new SecureCookie($salt,'random',time()+3600,'/','.domain.co.nz');
$C->Set('triton',$userid);
}

but how do I delete it from another function? for example//

function delete_cookie()
{
$C-Del("random")
}

doesn't work nor does setcookie('random',"", time()-3600) or unset($_COOKIE["random"]);

I tried creating the cookie again in the same function but I just get a 'division by 0' error.

thx