title: Where does v4l2_buffer->timestamp value starts counting?
body: <p>I am trying to use v4l2_buffer's timestamp value (type timeval) to synchronize images captured from a <em>UVC</em> webcam to external events.</p>
  
  <p>However the timestamp is not the same as the system time, or the up time, etc:</p>
  
  <pre><code>printf("image captured at %ld, %ld\n",
     buffer-&gt;timestamp.tv_sec, 
     buffer-&gt;timestamp.tv_usec);
  
  struct timeval tv;
  gettimeofday(&amp;tv, 0);
  printf("current time %ld, %ld\n", tv.tv_sec, tv.tv_usec);
  </code></pre>
  
  <p>Results in</p>
  
  <pre><code>image captured at 367746, 476270
  current time 1335083395, 11225
  </code></pre>
  
  <p>My uptime is 10 days.</p>