title: Initializing boost date to the first year of AD
body: <p>I would want to initialize a gregorian date like this</p>
  
  <p><code>boost::gregorian::date d = { 1, 1, 1 };</code></p>
  
  <p>but <code>year = 1</code> is not allowed.
  How can I create a day before Jesus was born ?</p>