Date control based on Asp.net and Javascript control_javascript skills

The effect of the control is as follows:

From left to right: month, day, year

.cs file initializes these three drop-down lists

Copy the code The code is as follows:

private void BindBirthDay(int day, int month, int year)
{
int dayNow = day;
int monNow = month;
int yearNow = year;
//binding Month
for (int i = 1; i <= 12; i )
{
ddlBirMon.Items.Add(new ListItem(i.ToString(), i.ToString()));
}
ddlBirMon.Items[monNow - 1].Selected = true;
//binding Day
int daysOfMonth = DateTime.DaysInMonth(yearNow, monNow);
for (int i = 1 ; i <= daysOfMonth; i )
{
ddlBirDay.Items.Add(new ListItem(i.ToString(), i.ToString()));
}
ddlBirDay.Items[ dayNow - 1].Selected = true;
//binding Year
for (int i = 20; i > 0; i--)
{
ddlBirYear.Items.Add(new ListItem ((yearNow - i).ToString(), (yearNow - i).ToString()));
}
for (int i = 0; i < 20; i )
{
ddlBirYear.Items.Add(new ListItem((yearNow i).ToString(), (yearNow i).ToString()));
}
ddlBirYear.Items.FindByValue(yearNow.ToString()). Selected = true;
}

The js code is as follows (written by myself, not guaranteed to be completely correct):

Copy code The code is as follows:

function ChangeDay() {
var month = document.getElementById("<%=ddlBirMon.ClientID %>");
var year = document.getElementById("<%=ddlBirYear.ClientID %>");
var day = document.getElementById("<%=ddlBirDay.ClientID %>");
if (month.selectedIndex == 3 || month.selectedIndex == 5 || month.selectedIndex == 8 || month.selectedIndex == 10) {
if (day.length == 31) {
if (day.options[30].selected == true) {
day.options[29].selected = true;
}
day.remove(30);
}
}
else{
while (day.length < 31) {
day.add(new Option(day.length 1,day.length 1));
}
}
if (month.selectedIndex == 1) {
if (day.length > 28) {
if (day.selectedIndex == 28) {
day.options[27].selected = true;
}
while (day.length > 28) {
day.remove(day.length - 1);
}
}
var sy = year.options [year.selectedIndex].value;
if ((sy % 4 == 0 && sy % 100 != 0) || (sy % 400==0)) {
day.add(new Option( "29", "29"));
}
}
}