https://www.future-labo.com/layout/fl/common/js/get_calender.js

1//日付のセレクトボックスを妥当な値にする
2//onChange="setDay('start_year','start_month','start_day');"
3function setDay(yearId,monthId,dayId){
4	dayObj = document.getElementById(dayId);
5	
6	var yearObj = document.getElementById(yearId);
7	var monthObj = document.getElementById(monthId);
8	var years = parseInt(yearObj.options[yearObj.selectedIndex].text);
9	var months = parseInt(monthObj.options[monthObj.selectedIndex].text);
10	if (years > 0 && months > 0) {
11		CalenderDwr.getMaxDate(years , months , setDayCallBack);
12	}
13} 
14
15function setDayCallBack(result){
16	var lastday = result
17	
18	var firstVal = dayObj.options[0].value;
19	var oldLength = dayObj.length;
20	var newLength = (firstVal < 0) ? lastday + 1 : lastday;
21	
22	if (dayObj.selectedIndex > newLength - 1) {
23		dayObj.selectedIndex = newLength - 1;
24	}
25	
26	dayObj.length = newLength;
27	for (var i = oldLength; i < newLength; i++) {
28		dayObj.options[i].value = (firstVal < 0) ? i : i + 1;
29		dayObj.options[i].text = (firstVal < 0) ? i : i + 1;
30	}
31}