现在世界通用的公历（阳历）也经过一个长期演变的过程。我们先看，公历每个月的日数是固定的："七前单大，八后双大"。也就是说，一、三、五、七、八、十、腊月（十二月）是31天，四、六、九、十一月是30天，只有二月，平年28天，闰年29天。
复制代码 代码如下:
<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="WebForm1.aspx.cs" Inherits="WebApplication1.WebForm1" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
<title></title>
</head>
<body>
<form runat="server">
<div>
<select name=YYYY onchange="YYYYMM(this.value)">
<option value="">选择 年</option>
</select>
<select name=MM onchange="MMDD(this.value)">
<option value="">选择 月</option>
</select>
<select name=DD>
<option value="">选择 日</option>
</select>
</div>
<asp:HiddenField runat="server" />
<asp:Button runat="server" Text="保存" />
</form>
</body>
</html>
<script language="JavaScript">
<!--
function getValue() {
var year = document.getElementById("YYYY").options[document.getElementById("YYYY").selectedIndex].value;
var month = document.getElementById("MM").options[document.getElementById("MM").selectedIndex].value;
var day = document.getElementById("DD").options[document.getElementById("DD").selectedIndex].value;
document.getElementById("hfValue").value = year+"-"+month+"-"+day;
}
window.onload=function() {
strYYYY = document.form1.YYYY.outerHTML;
strMM = document.form1.MM.outerHTML;
strDD = document.form1.DD.outerHTML;
MonHead = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
//先给年下拉框赋内容
var y = new Date().getFullYear();
var str = strYYYY.substring(0, strYYYY.length - 9);
for (var i = (y - 80); i < (y + 2); i++) //以今年为准，前30年，后30年
{
str += "<option value='" + i + "'> " + i + " 年" + "</option>\r\n";
}
document.form1.YYYY.outerHTML = str + "</select>";
//赋月份的下拉框
var str = strMM.substring(0, strMM.length - 9);
for (var i = 1; i < 13; i++) {
str += "<option value='" + i + "'> " + i + " 月" + "</option>\r\n";
}
document.form1.MM.outerHTML = str + "</select>";
document.form1.YYYY.value = y;
document.form1.MM.value = new Date().getMonth() + 1;
var n = MonHead[new Date().getMonth()];
if (new Date().getMonth() == 1 && IsPinYear(YYYYvalue)) n++;
writeDay(n); //赋日期下拉框
document.form1.DD.value = new Date().getDate();
}
function YYYYMM(str) //年发生变化时日期发生变化(主要是判断闰平年)
{
var MMvalue = document.form1.MM.options[document.form1.MM.selectedIndex].value;
if (MMvalue == "") {
DD.outerHTML = strDD;
return;
}
var n = MonHead[MMvalue - 1];
if (MMvalue == 2 && IsPinYear(str)) n++;
writeDay(n)
}
function MMDD(str) //月发生变化时日期联动
{
var YYYYvalue = document.form1.YYYY.options[document.form1.YYYY.selectedIndex].value;
if (str == "") {
DD.outerHTML = strDD;
return;
}
var n = MonHead[str - 1];
if (str == 2 && IsPinYear(YYYYvalue)) n++;
writeDay(n)
}
function writeDay(n) //据条件写日期的下拉框
{
var s = strDD.substring(0, strDD.length - 9);
for (var i = 1; i < (n + 1); i++) s += "<option value='" + i + "'> " + i + " 日" +
"</option>\r\n";
document.form1.DD.outerHTML = s + "</select>";