title: What is the canonical representation of zero in ISO 8601 durations?
body: <p><a href="http://en.wikipedia.org/wiki/ISO_8601#Durations" rel="noreferrer">http://en.wikipedia.org/wiki/ISO_8601#Durations</a></p>
  
  <p>It's not clear what the most correct representation of zero in ISO 8601 durations is.</p>
  
  <p>Possible candidates:</p>
  
  <pre><code>PT0S
  </code></pre>
  
  <p>This site:
  <a href="http://www.ostyn.com/standards/scorm/samples/ISOTimeForSCORM.htm" rel="noreferrer">http://www.ostyn.com/standards/scorm/samples/ISOTimeForSCORM.htm</a>
  says</p>
  
  <pre><code>PT0H0M0S
  </code></pre>
  
  <p>Or probably the simplest is</p>
  
  <pre><code>P
  </code></pre>
  
  <p>But what is most correct? Is there a canonical zero duration representation?</p>