alues can be of an arbitrary size.
 Implementations may chose not to or be incapable of supporting arbitrarily large and/or small values.
 An <CODE>UnsupportedOperationException</CODE> will be thrown with a message indicating implementation limits
 if implementation capacities are exceeded.</p>
 
 <p>A <code>null</code> value indicates that field is not set.</p><DD><DL>
<DT><B>Parameters:</B><DD><CODE>isPositive</CODE> - Set to <code>false</code> to create a negative duration. When the length
   of the dura