title: Difference between two time_point instances is not a duration?
body: <p>I don't understand why this code chokes with g++ 4.7.2:</p>
  
  <pre><code>#include &lt;chrono&gt;
  
  main ()
  {
      std::chrono::system_clock::time_point t1, t2 ;
      std::chrono::seconds delay ;
  
      t1 = std::chrono::system_clock::time_point::max () ;
      t2 = std::chrono::system_clock::now () ;
      delay = t1 - t2 ;
      // t1 = t2 + delay ;
      // t1 = t2 - delay ;
  }
  </code></pre>
  
  <p>with the error:</p>
  
  <pre><code>test.cc: In function ‘int main()’:
  test.cc:10:18: error: no match for ‘operator=’ in ‘delay = std::chrono::operator,&lt;std::chrono::system_clock, std::chrono::duration&lt;long int, std::ratio&lt;1l, 1000000l&gt; &gt;, std::chrono::duration&lt;long int, std::ratio&lt;1l, 1000000l&gt; &gt; &gt;((*(const std::chrono::time_point&lt;std::chrono::system_clock, std::chrono::duration&lt;long int, std::ratio&lt;1l, 1000000l&gt; &gt; &gt;*)(&amp; t1)), (*(const std::chrono::time_point&lt;std::chrono::system_clock, std::chrono::duration&lt;long int, std::ratio&lt;1l, 1000000l&gt; &gt; &gt;*)(&amp; t2)))’
  </code></pre>
  
  <p>It seemed to me that "time_point - time_point" gives a "duration".</p>