title: difftime returns incorrect value in MinGW
body: <p>I'm trying to calculate difference between two <code>time_t</code>.<br>
  but <code>difftime</code> returns its first parameter instead of the difference!<br>
  My code is:  </p>
  
  <pre><code> #include &lt;windows.h&gt;
   #include &lt;stdio.h&gt;
   #include &lt;time.h&gt;
   #include &lt;unistd.h&gt;
  
   int main(){
       time_t etime_t,now_t;
       double time_diff;
       now_t=1388525484L;
       etime_t=1389338644L;
       time_diff=difftime(now_t,etime_t);
       printf("%f",time_diff);
   }
  </code></pre>
  
  <p>And it prints:  </p>
  
  <pre><code>1388525484.000000
  </code></pre>
  
  <p>I'm compiling with <code>GCC</code> (in <code>MinWG</code>)<br>
  What is the problem?</p>