title: Build/parse UTC date using standard library
body: <p>I'm having troubles building a <strong>UTC</strong> date in c++ using the standard library.
  For example (any is fine): </p>
  
  <ul>
  <li>building from separate variables containing day month year...</li>
  </ul>
  
  <p>or parsing </p>
  
  <ul>
  <li><blockquote>
    <p>01/01/1970 00:00:00 UTC</p>
  </blockquote></li>
  <li><blockquote>
    <p>01/01/1970 00:00:00 +0000</p>
  </blockquote></li>
  <li><blockquote>
    <p>01/01/1970 00:00:00 # (implicitely in UTC)</p>
  </blockquote></li>
  </ul>
  
  <p>Conditions:</p>
  
  <ul>
  <li>The timezone string does not need to be interpreted but I need to be sure the date is interpreted as UTC. </li>
  <li>the date is always correctly formated</li>
  <li>any formatting is accepted as long as it does the job</li>
  <li>should be portable (UNIX variants and Windows)</li>
  <li>avoid external libraries if possible</li>
  </ul>
  
  <p>The best candidates so far are:</p>
  
  <ul>
  <li><code>std::time_get</code> but specification doesn't say anything about the timezone (for what I understood of it :-) ).</li>
  <li>boost's <code>date_time</code>, but its a pretty heavy dependency (mentioned in <a href="https://stackoverflow.com/questions/3547510/c-library-unix-to-parse-date-time-string-including-timezones">C++ library (unix) to parse date/time string Including timezones</a>, but the answer predates C++11)</li>
  <li>unix <code>strptime</code> is not portable on windows</li>
  <li>messing around with <code>std::tm</code> directly, but on my system (glibc), it has non-standard undocumented fields (namely tm_zone ...). I suppose I could initialize it with <code>gmtime(0)</code> then modify the days, month, year fields.</li>
  </ul>
  
  <p>How can I build an <code>std::chrono::time_point::time_point</code> from the UTC date?</p>