title: %D field in Apache access logs - first or last byte?
body: <p>The Apache Httpd manual has a section on <a href="http://httpd.apache.org/docs/current/mod/mod_log_config.html#formats" rel="noreferrer">custom access log formats</a>.  One of these options is the <code>%D</code> field, which is documented as</p>
  
  <blockquote>
    <p>The time taken to serve the request, in microseconds.</p>
  </blockquote>
  
  <p>Can anyone tell me what exactly this is measuring?  Is it time-to-first-byte, or time-to-last-byte, for example, or something more complex than that?  </p>
  
  <p>I need this is demonstrate compliance to performance requirements, and I want to know exactly what's being measured here.</p>