title: C++ timestamp and diffTime
body: <p>I'm trying to know the timestamp when I receive a message.
  I should check if it has been N seconds of receiving the last message.
  I'm using <code>time_t</code>.
  I take the current time like this: <code>time (&amp; myTime);</code></p>
  
  <p>how can I check if they spent more than N seconds?</p>
  
  <p>Is there a way to know the number of seconds (timestamp) of a total time_t?</p>