title: How can I tell the "current age" of a cached page?
body: <p>I'm wondering how the browser determines whether a cached resource has expired or not.</p>
  
  <p>Assume that I have set the <code>max-age</code> header to 300. I made a request at 14:00, 3 minutes later I made another request to the same resource. So how can the browser tell the resource haven't expired (the <code>current age</code> which is 180 is less than the <code>max-age</code>)? Does the browser hold a "expiry date" or "current age" for every requested resource? If so how can I inspect the "current age" at the time I made the request?</p>