function updateNumberOfDays(day_selector, month_selector, year_selector) {
    var month_index = month_selector.selectedIndex ;
	var year = year_selector.options[year_selector.selectedIndex].value ;
	var curday = day_selector.selectedIndex ;
    var date = new Date(year, month_index+1, 0) ;
	var dd = date.getDate() ;

	for (var i = 0; i < day_selector.length+1; i++) {
	  day_selector.options[0] = null ;
	}
	
	for (var i = 0; i < dd; i++) {
		day_selector.options[i] = new Option(i+1, i+1) ;
	}
	
	if (curday < day_selector.length) {
    	day_selector.options[curday].selected = true ;
	}
	
}