title: "1970-01-01 00:00:00" expires header
body: <p>I am checking files that are cached in my firefox browser and I note a load of them have an expires (<code>Expires:</code>) of:</p>
  
  <pre><code>"1970-01-01 00:00:00" expires header
  </code></pre>
  
  <p>This must be a common setting with a common meaning. What exactly?  No expiration set?</p>
  
  <p>Any tips?</p>