title: How to keep track of non empty field in Joda date buffer
body: <p>I'm writing a set of objects that should be able to alter fields in a <a href="http://www.google.it/url?sa=t&amp;rct=j&amp;q=&amp;esrc=s&amp;source=web&amp;cd=1&amp;ved=0CH8QFjAA&amp;url=http://joda-time.sourceforge.net/&amp;ei=HxCmT4qyBuPl4QSq6PiGCQ&amp;usg=AFQjCNH25QvdckCfXWVVnS-A2r46o4wp6g" rel="nofollow">Joda Time</a> MutableDateTime instance. </p>
  
  <p>Each object is applied in sequence to the buffer, and when all that set is applied, a complete valid MutableDateTime will be built.</p>
  
  <p>Each instance have to be able to know which date time fields has already been set by other instances in the set.</p>
  
  <p>I get stucked because I get following problems:</p>
  
  <ol>
  <li>How to create the MutableDateTime instance with empty values in all
  date time fields in order to use it as the initial value to build?</li>
  <li>How could I know if some field of MutableDateTime has been set?</li>
  </ol>
  
  <p>MutableDateTime internally keep track it's data in a long instance field initialized to number of milliseconds elapsed from start of era to now. It thus has all field already set to some value.</p>
  
  <p>Do you know if MutableDateTime has some concept of an empty value?</p>
  
  <p><strong>Edit:</strong></p>
  
  <p>as I show in my response, I develop a solution using a manager class as Vladimir suggested.</p>