// cambia el select de los dias dependiendo del mes seleccionado.
function change_month ( month_number )
{
	var days = 31;
	if ( month_number == 4 ||
		 month_number == 6 || 
		 month_number == 9 ||
		 month_number == 11) 
	{
		days = 30;
	} 
	else if ( month_number == 1 || 
			  month_number == 3 ||
			  month_number == 5 ||
			  month_number == 7 ||
			  month_number == 8 ||
			  month_number == 10 ||
			  month_number == 12) 
	{
		days = 31;	
	} else {
		days = 28;	
	}
	
	// Borro los option del select de dias y los creo de nuevo con la cantidad correcta.
	var day_select = $('fecnac');
	var day_options = day_select.descendants();
	var selected_day = day_select.selectedIndex + 1;
	
	// borramos su contenido... 
	for ( var i = 1; i <= day_options.length; i ++ )
	{
		day_select.firstDescendant().remove();
	}
	// lo rellenamos con la cantidad correcta...
	for ( var i = 1; i <= days; i ++ )
	{
		option = document.createElement('option');
		option.value = i;
		option.innerHTML = i;
		if ( i == selected_day ) {
			option.selected = true;	
		}
		day_select.appendChild(option);	
	}

}