Date triple drop-down box (forwarded from network + organized)

Reprinted and sorted out:

Code
<Script language = "javascript" type = "text/javascript">
// For birthday


Function YYYYMMDDstart ()
{
MonHead = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31];

// Add content to the annual drop-down box first
Var y = new Date (). getFullYear ();
For (var I = (y-100); I <(y + 1); I ++) // take this year as standard, first 30 years, last 30 years
Year. options. add (new Option ("" + I + "year", I ));

// Drop-down box for month assignment
For (var I = 1; I <13; I ++)
Month. options. add (new Option ("" + I + "month", I ));

Year. value = y;
Month. value = new Date (). getMonth () + 1;
Var n = MonHead [new Date (). getMonth ()];
If (new Date (). getMonth () = 1 & IsPinYear (YYYYvalue) n ++;
WriteDay (n); // Date drop-down box
Day. value = new Date (). getDate ();
}

Function YYYYDD (str) // The date when the year changes (mainly to determine the year of the week)
{
Var MMvalue = month. value;
If (MMvalue = "") {var e = day; optionsClear (e); return ;}
Var n = MonHead [MMvalue-1];
Var dayValue = day. value;
If (MMvalue = 2 & IsPinYear (str) n ++;
WriteDay (n );
// If the selected date is within this range, reset the date.
If (dayValue <= n );
Day. value = dayValue;
}
Function MMDD (str) // Date association when the month changes
{
Var YYYYvalue = year. value;
If (YYYYvalue = "") {var e = day; optionsClear (e); return ;}
Var n = MonHead [str-1];
Var dayValue = day. value;
If (str = 2 & IsPinYear (YYYYvalue) n ++;
WriteDay (n );
If (dayValue <= n );
Day. value = dayValue;
}
Function writeDay (n) // drop-down list of write dates based on conditions
{
Var e = day; optionsClear (e );
For (var I = 1; I <(n + 1); I ++)
E. options. add (new Option ("" + I + "", I ));
}
Function IsPinYear (year) // determines whether to renew the year
{
Return (0 = year % 4 & (year % 100! = 0 | year % 400 = 0 ));
}
Function optionsClear (e)
{
E. options. length = 1;
}

</Script>

Html code:

<Td class = "font_R"> birthday </td>
<Td>
<Select id = "ddlYear" runat = "server" onchange = "YYYYDD (this. value)">
<Option value = "" selected = "selected"> select year </option>
</Select>
<Select id = "ddlMonth" runat = "server" onchange = "MMDD (this. value)">
<Option value = "" selected = "selected"> select month </option>
</Select>
<Select id = "ddlDay" runat = "server">
<Option value = "" runat = "server" selected = "selected"> select day </option>
</Select>
</Td>

……

        var year, month, day;
        year = document.getElementById('<%=ddlYear.ClientID %>');
        month = document.getElementById('<%=ddlMonth.ClientID %>');
        day = document.getElementById('<%=ddlDay.ClientID %>');

        YYYYMMDDstart();
"<script type='text/javascript' language ='javascript' >"+
            "year.value={0};YYYYDD({0});year.value={0};month.value={1};day.value={2}</script>", dt.Year.ToString(), dt.Month.ToString(), dt.Day.ToString());