var date_arr = new Array;

date_arr[1]=31;
date_arr[2]=28;
date_arr[3]=31;
date_arr[4]=30;
date_arr[5]=31;
date_arr[6]=30;
date_arr[7]=31;
date_arr[8]=30;
date_arr[9]=30;
date_arr[10]=31;
date_arr[11]=30;
date_arr[12]=31;

function update_days(prefix)
{
  var fieldname = prefix + 'month';
  var daysname = prefix +'day';

	var monthindex = document.getElementById(fieldname).selectedIndex;   
	var month = document.getElementById(fieldname).options[monthindex].value; 
	
	var dayindex = document.getElementById(daysname).selectedIndex;   
	var day = document.getElementById(daysname).options[dayindex].value; 
	
	document.getElementById(daysname).innerHTML='';
	
	for(i=1; i<=date_arr[month];i++)
  {
  	newopt = document.createElement('option');
  	newopt.value = i;
  	newopt.innerHTML = i;
  	if (i == day)
  	{
  		newopt.selected = true;
  	}
  	document.getElementById(daysname).appendChild(newopt);
  }
}