title: What does 'PT' prefix stand for in Duration?
body: <p>I am trying to use the <code>Duration</code> class instead of <code>long</code>. 
  It has superior literal syntax. I like its flexibility, though it looks weird.</p>
  
  <p>"PT10S" means 10 seconds, what is the problem to accept "10 seconds"?!
  Okay never mind.</p>
  
  <p>I am just curious why PT prefix has been chosen (not "DU" e.g.) and why any prefix is better here rather than nothing?</p>