Javascript getting year from date

Hi, 
I need to set the year from the birthdate, month and day is setting the values correctly while the year is reading it right but don't set it. why ?
below is my script,
thank you 


//==== Start OF: GetBirthdate=============================================================
function GetBirthdate()
{
debugger;
  if(Xrm.Page.getAttribute("new_birthdate").getValue() != null)
  {

var dateFieldValue= Xrm.Page.getAttribute('new_birthdate').getValue();
var year = dateFieldValue.getFullYear();
var month = dateFieldValue.getMonth()+1;
var day = dateFieldValue.getDate();

//var dateOnly = new Date(year,month+1,day);

Xrm.Page.getAttribute("new_birthday").setValue(day);
Xrm.Page.getAttribute("new_birthmonth").setValue(month);
Xrm.Page.getAttribute("new_birthyear").setValue(year);


  }
  else
  {
	Xrm.Page.getAttribute("new_birthday").setValue(null);
	Xrm.Page.getAttribute("new_birthmonth").setValue(null);
	Xrm.Page.getAttribute("new_birthyear").setValue(null);
  }
}// end of function
//==== END OF: Fill Birthday,BirthMonth and Birthyear if birthdate is filled