title: Date from string boost::gregorian
body: <p>Hi I have two strings:</p>
  
  <pre><code>entertime "2014-03-06T09:35:36Z" 
  exittime "2014-03-06T09:38:36Z" 
  </code></pre>
  
  <p>I want to be able to return the difference, which in this case should return 3min.</p>
  
  <p>My method looks like this:</p>
  
  <pre><code> boost::gregorian::date returnTime(std::string entertime, std::string exittime){
   boost::gregorian::date denter = boost::gregorian::from_string(entertime);
   boost::gregorian::date dexit = boost::gregorian::from_string(entertime);
   boost::gregorian::date dresult = dexit-denter;
  </code></pre>
  
  <p>Is there any way to get this time difference?</p>
  
  <p>Thank you in advance!</p>