title: expire cookie - magical number -86400
body: <p>I'm reviewing the code of my framework which deletes cookie on client and here is the piece of code where it does it:</p>
  
  <p><code>return setcookie($name, NULL, -86400, Cookie::$path, Cookie::$domain, Cookie::$secure, Cookie::$httponly);</code></p>
  
  <p>I'm particularly interested in why the number is <code>-86400</code> seconds? Is it just random number or it has any special meaning in some convention? I can expire cookie with <code>-1</code>, can't I?</p>