title: HTTP Expires header values "0" and "-1"
body: <p>What is the difference between <code>Expires: 0</code> and <code>Expires: -1</code> in the HTTP response header? <a href="https://www.rfc-editor.org/rfc/rfc2616#section-14.21" rel="nofollow noreferrer">RFC 2616</a> defines <em>invalid date formats, especially including the value &quot;0&quot;</em> as <em>already expired</em>. However, some servers (e.g. <a href="http://www.google.de" rel="nofollow noreferrer">www.google.de</a>) reply with <code>Expires: -1</code>.</p>
  <p>Is there an advantage over using <code>-1</code> over <code>0</code> or is this even required for some broken HTTP clients?</p>