date.js代码：
/*=======Calendar.js=======By Jiang Hongbin=======*/

var months = new Array("一月", "二月", "三月", "四月", "五月", "六月", "七月", "八月", "九月", "十月", "十一月", "十二月");
var daysInMonth = new Array(31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31);
var days = new Array("日","一", "二", "三", "四", "五", "六");
var today;

document.writeln("<div id='Calendar' style='position:absolute; z-index:1; visibility: hidden; filter:\"progid:DXImageTransform.Microsoft.Shadow(direction=135,color=#999999,strength=3)\"'></div>");

function getDays(month, year)
{
 //下面的这段代码是判断当前是否是闰年的
 if (1 == month)
  return ((0 == year % 4) && (0 != (year % 100))) || (0 == year % 400) ? 29 : 28;
 else
  return daysInMonth[month];
}

function getToday()
{
 //得到今天的年,月,日
 this.now = new Date();
 this.year = this.now.getFullYear();
 this.month = this.now.getMonth();
 this.day = this.now.getDate();
}

function getStringDay(str)
{
 //得到输入框的年,月,日
 var str=str.split("-")
 
 this.now = new Date(parseFloat(str[0]),parseFloat(str[1])-1,parseFloat(str[2]));
 this.year = this.now.getFullYear();
 this.month = this.now.getMonth();
 this.day = this.now.getDate();
}

function newCalendar() {
 var parseYear = parseInt(document.all.Year.options[document.all.Year.selectedIndex].value);
 var newCal = new Date(parseYear, document.all.Month.selectedIndex, 1);
 var day = -1;
 var startDay = newCal.getDay();
 var daily = 0;
 
 if ((today.year == newCal.getFullYear()) &&(today.month == newCal.getMonth()))
  day = today.day;
  
 var tableCal = document.all.calendar;
 var intDaysInMonth =getDays(newCal.getMonth(), newCal.getFullYear());
 
 for (var intWeek = 1; intWeek < tableCal.rows.length;intWeek++)
  for (var intDay = 0;intDay < tableCal.rows[intWeek].cells.length;intDay++)
  {
   var cell = tableCal.rows[intWeek].cells[intDay];
   if ((intDay == startDay) && (0 == daily))
    daily = 1;
    
   if(day==daily) //今天，调用今天的Class
   {
    cell.style.background='#6699CC';
    cell.style.color='#FFFFFF';
    //cell.style.fontWeight='bold';
   }
   else if(intDay==6) //周六
    cell.style.color='green';
   else if (intDay==0) //周日
    cell.style.color='red';
   
   if ((daily > 0) && (daily <= intDaysInMonth))
   {
    cell.innerText = daily;
    daily++;
   }
   else
    cell.innerText = "";
  }
}

function GetDate(InputBox)
{
 var sDate;
 //这段代码处理鼠标点击的情况
 if (event.srcElement.tagName == "TD")
  if (event.srcElement.innerText != "")
  {
   sDate = document.all.Year.value + "-" + document.all.Month.value + "-" + event.srcElement.innerText;
   eval("document.all."+InputBox).value=sDate;
   HiddenCalendar();
  }
}

function HiddenCalendar()
{
 //关闭选择窗口
 document.all.Calendar.style.visibility='hidden';
}

function ShowCalendar(InputBox)
{
 var x,y,intLoop,intWeeks,intDays;
 var DivContent;
 var year,month,day;
 //var o=document.getElementById(InputBox);
 var o=InputBox;
 var oid=o.id;
 var thisyear; //真正的今年年份
 
 if(!oid)oid=o.name;
 
 thisyear=new getToday();
 thisyear=thisyear.year;
 
 today = o.value;
 if(isDate(today))
  today = new getStringDay(today);
 else
  today = new getToday();
 
 //显示的位置
 x=o.offsetLeft;
 y=o.offsetTop;
 while(o=o.offsetParent)
 {
  x+=o.offsetLeft;
  y+=o.offsetTop;
 }
 document.all.Calendar.style.left=x+2;
 document.all.Calendar.style.top=y+20;
 document.all.Calendar.style.visibility="visible";
 
 //下面开始输出日历表格(border-color:#9DBAF7)
 DivContent="<table border='0' cellspacing='0' style='border:1px solid #0066FF; background-color:#EDF2FC'>";
 DivContent+="<tr>";
 DivContent+="<td style='border-bottom:1px solid #0066FF; background-color:#C7D8FA'>";
 
 //年
 DivContent+="<select name='Year' id='Year' onChange='newCalendar()' style='font-family:Verdana; font-size:12px'>";
 for (intLoop = thisyear - 100; intLoop < (thisyear + 2); intLoop++)
  DivContent+="<option value= " + intLoop + " " + (today.year == intLoop ? "Selected" : "") + ">" + intLoop + "</option>";
 DivContent+="</select>";
 
 //月
 DivContent+="<select name='Month' id='Month' onChange='newCalendar()' style='font-family:Verdana; font-size:12px'>";
 for (intLoop = 0; intLoop < months.length; intLoop++)
  DivContent+="<option value= " + (intLoop + 1) + " " + (today.month == intLoop ? "Selected" : "") + ">" + months[intLoop] + "</option>";
 DivContent+="</select>";
 
 DivContent+="</td>";
 
 DivContent+="<td style='border-bottom:1px solid #0066FF; background-color:#C7D8FA; font-weight:bold; font-family:Wingdings 2,Wingdings,Webdings; font-size:16px; padding-top:2px; color:#4477FF; cursor:hand' align='center' title='关闭' onClick='javascript:HiddenCalendar()'>S</td>";
 DivContent+="</tr>";
 
 DivContent+="<tr><td align='center' colspan='2'>";
 DivContent+="<table id='calendar' border='0' width='100%'>";
 
 //星期
 DivContent+="<tr>";
 for (intLoop = 0; intLoop < days.length; intLoop++)
  DivContent+="<td align='center' style='font-size:12px'>" + days[intLoop] + "</td>";
 DivContent+="</tr>";
 
 //天
 for (intWeeks = 0; intWeeks < 6; intWeeks++)
 {
  DivContent+="<tr>";
  for (intDays = 0; intDays < days.length; intDays++)
   DivContent+="<td onClick='GetDate(\"" + oid + "\")' style='cursor:hand; border-right:1px solid #BBBBBB; border-bottom:1px solid #BBBBBB; color:#215DC6; font-family:Verdana; font-size:12px' align='center'></td>";
  DivContent+="</tr>";
 }
 DivContent+="</table></td></tr></table>";

 document.all.Calendar.innerHTML=DivContent;
 newCalendar();
}

function isDate(dateStr)
{
 var datePat = /^(\d{4})(\-)(\d{1,2})(\-)(\d{1,2})$/;
 var matchArray = dateStr.match(datePat);
 if (matchArray == null) return false;
 var month = matchArray[3];
 var day = matchArray[5];
 var year = matchArray[1];
 if (month < 1 || month > 12) return false;
 if (day < 1 || day > 31) return false;
 if ((month==4 || month==6 || month==9 || month==11) && day==31) return false;
 if (month == 2)
 {
  var isleap = (year % 4 == 0 && (year % 100 != 0 || year % 400 == 0));
  if (day > 29 || (day==29 && !isleap)) return false;
 }
 return true;
}

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<script type="text/javascript" src="js/date.js"></script>
<input id="date" onClick="javascript:ShowCalendar(this)"><input type="button" value="选择" onClick="javascript:ShowCalendar(date)"><br>