var winCal;
var dtToday=new Date();
var Cal;
var docCal;
var MonthName=["January", "February", "March", "April", "May", "June","July",
    "August", "September", "October", "November", "December"];
var WeekDayName=["Sunday","Monday","Tuesday","Wednesday","Thursday","Friday","Saturday"];    
var exDateTime;//Existing Date and Time

//Configurable parameters
var cnTop="200";//top coordinate of calendar window.
var cnLeft="500";//left coordinate of calendar window
var WindowTitle ="DateTime Picker";//Date Time Picker title.
var WeekChar=2;//number of character for week day. if 2 then Mo,Tu,We. if 3 then Mon,Tue,Wed.
var CellWidth=20;//Width of day cell.
var DateSeparator="/";//Date Separator, you can change it to "/" if you want.
var TimeMode=24;//default TimeMode value. 12 or 24

var ShowLongMonth=true;//Show long month name in Calendar header. example: "January".
var ShowMonthYear=true;//Show Month and Year in Calendar header.
var MonthYearColor="#cc0033";//Font Color of Month and Year in Calendar header.
var WeekHeadColor="#0099CC";//Background Color in Week header.
var SundayColor="#6699FF";//Background color of Sunday.
var SaturdayColor="#CCCCFF";//Background color of Saturday.
var WeekDayColor="white";//Background color of weekdays.
var FontColor="blue";//color of font in Calendar day cell.
var TodayColor="#FFFF33";//Background color of today.
var SelDateColor="#FFFF99";//Backgrond color of selected date in textbox.
var YrSelColor="#cc0033";//color of font of Year selector.
var ThemeBg="";//Background image of Calendar window.
//end Configurable parameters
//end Global variable

function NewCal(pCtrl,pFormat,pShowTime,pTimeMode)
{
    Cal=new Calendar(dtToday);
    if ((pShowTime!=null) && (pShowTime))
    {
        Cal.ShowTime=true;
        if ((pTimeMode!=null) &&((pTimeMode=='12')||(pTimeMode=='24')))
        {
            TimeMode=pTimeMode;
        }        
    }    
    if (pCtrl!=null)
        Cal.Ctrl=pCtrl;
    if (pFormat!=null)
        Cal.Format=pFormat.toUpperCase();
    
    exDateTime=document.getElementById(pCtrl).value;
    if (exDateTime!="")//Parse Date String
    {
        var Sp1;//Index of Date Separator 1
        var Sp2;//Index of Date Separator 2
        var tSp1;//Index of Time Separator 1
        var tSp1;//Index of Time Separator 2
        var strMonth;
        var strDate;
        var strYear;
        var intMonth;
        var YearPattern;
        var strHour;
        var strMinute;
        var strSecond;
        //parse month
        Sp1=exDateTime.indexOf(DateSeparator,0)
        Sp2=exDateTime.indexOf(DateSeparator,(parseInt(Sp1)+1));
        
        if ((Cal.Format.toUpperCase()=="DDMMYYYY") || (Cal.Format.toUpperCase()=="DDMMMYYYY"))
        {
            strMonth=exDateTime.substring(Sp1+1,Sp2);
            strDate=exDateTime.substring(0,Sp1);
        }
        else if ((Cal.Format.toUpperCase()=="MMDDYYYY") || (Cal.Format.toUpperCase()=="MMMDDYYYY"))
        {
            strMonth=exDateTime.substring(0,Sp1);
            strDate=exDateTime.substring(Sp1+1,Sp2);
        }
        if (isNaN(strMonth))
            intMonth=Cal.GetMonthIndex(strMonth);
        else
            intMonth=parseInt(strMonth,10)-1;    
        if ((parseInt(intMonth,10)>=0) && (parseInt(intMonth,10)<12))
            Cal.Month=intMonth;
        //end parse month
        //parse Date
        if ((parseInt(strDate,10)<=Cal.GetMonDays()) && (parseInt(strDate,10)>=1))
            Cal.Date=strDate;
        //end parse Date
        //parse year
        strYear=exDateTime.substring(Sp2+1,Sp2+5);
        YearPattern=/^\d{4}$/;
        if (YearPattern.test(strYear))
            Cal.Year=parseInt(strYear,10);
        //end parse year
        //parse time
        if (Cal.ShowTime==true)
        {
            tSp1=exDateTime.indexOf(":",0)
            tSp2=exDateTime.indexOf(":",(parseInt(tSp1)+1));
            strHour=exDateTime.substring(tSp1,(tSp1)-2);
            Cal.SetHour(strHour);
            strMinute=exDateTime.substring(tSp1+1,tSp2);
            Cal.SetMinute(strMinute);
            strSecond=exDateTime.substring(tSp2+1,tSp2+3);
            Cal.SetSecond(strSecond);
        }    
    }
    winCal=window.open("","DateTimePicker","toolbar=0,status=0,menubar=0,fullscreen=no,width=195,height=245,resizable=0,
top="+cnTop+",left="+cnLeft);
    docCal=winCal.document;
    RenderCal();
}

function RenderCal()
{
    var vCalHeader;
    var vCalData;
    var vCalTime;
    var i;
    var j;
    var SelectStr;
    var vDayCount=0;
    var vFirstDay;

    docCal.open();
    docCal.writeln("<html><head><title>"+WindowTitle+"</title>");
    docCal.writeln("&lt;script>var winMain=window.opener;</script>");
    docCal.writeln("</head><body background='"+ThemeBg+"' link="+FontColor+" vlink="+FontColor+"><form name='Calendar'>");

    vCalHeader="<table border=1 cellpadding=1 cellspacing=1 width='100%' align=\"center\" valign=\"top\">\n";
    //Month Selector
    vCalHeader+="<tr>\n<td colspan='7'><table border=0 width='100%' cellpadding=0 cellspacing=0><tr><td align='left'>\n";
    vCalHeader+="<select name=\"MonthSelector\" onChange=\"java script:winMain.Cal.SwitchMth(this.selectedIndex);winMain.RenderCal();\">\n";
    for (i=0;i<12;i++)
    {
        if (i==Cal.Month)
            SelectStr="Selected";
        else
            SelectStr="";    
        vCalHeader+="<option "+SelectStr+" value >"+MonthName[i]+"\n";
    }
    vCalHeader+="</select></td>";
    //Year selector
    vCalHeader+="\n<td align='right'><a href=\"java script:winMain.Cal.DecYear();winMain.RenderCal()\"><b><font color=\""+YrSelColor+"\"><</font></b></a><font face=\"Verdana\" color=\""+YrSelColor+"\" size=2><b> "+Cal.Year+" </b></font><a href=\"java script:winMain.Cal.IncYear();winMain.RenderCal()\"><b><font color=\""+YrSelColor+"\">></font></b></a></td></tr></table></td>\n";    
    vCalHeader+="</tr>";
    //Calendar header shows Month and Year
    if (ShowMonthYear)
        vCalHeader+="<tr><td colspan='7'><font face='Verdana' size='2' align='center' color='"+MonthYearColor+"'><b>"+Cal.GetMonthName(ShowLongMonth)+" "+Cal.Year+"</b></font></td></tr>\n";
    //Week day header
    vCalHeader+="<tr bgcolor="+WeekHeadColor+">";
    for (i=0;i<7;i++)
    {
        vCalHeader+="<td align='center'><font face='Verdana' size='2'>"+WeekDayName[i].substr(0,WeekChar)+"</font></td>";
    }
    vCalHeader+="</tr>";    
    docCal.write(vCalHeader);
    
    //Calendar detail
    CalDate=new Date(Cal.Year,Cal.Month);
    CalDate.setDate(1);
    vFirstDay=CalDate.getDay();
    vCalData="<tr>";
    for (i=0;i<vFirstDay;i++)
    {
        vCalData=vCalData+GenCell();
        vDayCount=vDayCount+1;
    }
    for (j=1;j<=Cal.GetMonDays();j++)
    {
        var strCell;
        vDayCount=vDayCount+1;
        if ((j==dtToday.getDate())&&(Cal.Month==dtToday.getMonth())&&(Cal.Year==dtToday.getFullYear()))
            strCell=GenCell(j,true,TodayColor);//Highlight today's date
        else
        {
            if (j==Cal.Date)
            {
                strCell=GenCell(j,true,SelDateColor);
            }
            else
            {    
                if (vDayCount%7==0)
                    strCell=GenCell(j,false,SaturdayColor);
                else if ((vDayCount+6)%7==0)
                    strCell=GenCell(j,false,SundayColor);
                else
                    strCell=GenCell(j,null,WeekDayColor);
            }        
        }                        
        vCalData=vCalData+strCell;

        if((vDayCount%7==0)&&(j<Cal.GetMonDays()))
        {
            vCalData=vCalData+"</tr>\n<tr>";
        }
    }
    docCal.writeln(vCalData);    
    //Time picker
    if (Cal.ShowTime)
    {
        var showHour;
        showHour=Cal.getShowHour();        
        vCalTime="<tr>\n<td colspan='7' align='center'>";
        vCalTime+="<input type='text' name='hour' maxlength=2 size=1 style=\"WIDTH: 22px\" value="+showHour+" onchange=\"java script:winMain.Cal.SetHour(this.value)\">";
        vCalTime+=" : ";
        vCalTime+="<input type='text' name='minute' maxlength=2 size=1 style=\"WIDTH: 22px\" value="+Cal.Minutes+" onchange=\"java script:winMain.Cal.SetMinute(this.value)\">";
        vCalTime+=" : ";
        vCalTime+="<input type='text' name='second' maxlength=2 size=1 style=\"WIDTH: 22px\" value="+Cal.Seconds+" onchange=\"java script:winMain.Cal.SetSecond(this.value)\">";
        if (TimeMode==12)
        {
            var SelectAm =(parseInt(Cal.Hours,10)<12)? "Selected":"";
            var SelectPm =(parseInt(Cal.Hours,10)>=12)? "Selected":"";

            vCalTime+="<select name=\"ampm\" onchange=\"java script:winMain.Cal.SetAmPm(this.options[this.selectedIndex].value);\">";
            vCalTime+="<option "+SelectAm+" value=\"AM\">AM</option>";
            vCalTime+="<option "+SelectPm+" value=\"PM\">PM<option>";
            vCalTime+="</select>";
        }    
        vCalTime+="\n</td>\n</tr>";
        docCal.write(vCalTime);
    }    
    //end time picker
    docCal.writeln("\n</table>");
    docCal.writeln("</form></body></html>");
    docCal.close();
}

function GenCell(pValue,pHighLight,pColor)//Generate table cell with value
{
    var PValue;
    var PCellStr;
    var vColor;
    var vHLstr1;//HighLight string
    var vHlstr2;
    var vTimeStr;
    
    if (pValue==null)
        PValue="";
    else
        PValue=pValue;
    
    if (pColor!=null)
        vColor="bgcolor=\""+pColor+"\"";
    else
        vColor="";    
    if ((pHighLight!=null)&&(pHighLight))
        {vHLstr1="color='red'><b>";vHLstr2="</b>";}
    else
        {vHLstr1=">";vHLstr2="";}    
    
    if (Cal.ShowTime)
    {
        vTimeStr="winMain.document.getElementById('"+Cal.Ctrl+"').value+=' '+"+"winMain.Cal.getShowHour()"+"+':'+"+"winMain.Cal.Minutes"+"+':'+"+"winMain.Cal.Seconds";
        if (TimeMode==12)
            vTimeStr+="+' '+winMain.Cal.AMorPM";
    }    
    else
        vTimeStr="";        
    PCellStr="<td "+vColor+" width="+CellWidth+" align='center'><font face='verdana' size='2'"+vHLstr1+"<a href=\"java script:winMain.document.getElementById('"+Cal.Ctrl+"').value='"+Cal.FormatDate(PValue)+"';"+vTimeStr+";window.close();\">"+PValue+"</a>"+vHLstr2+"</font></td>";
    return PCellStr;
}

function Calendar(pDate,pCtrl)
{
    //Properties
    this.Date=pDate.getDate();//selected date
    this.Month=pDate.getMonth();//selected month number
    this.Year=pDate.getFullYear();//selected year in 4 digits
    this.Hours=pDate.getHours();    
    
    if (pDate.getMinutes()<10)
        this.Minutes="0"+pDate.getMinutes();
    else
        this.Minutes=pDate.getMinutes();
    
    if (pDate.getSeconds()<10)
        this.Seconds="0"+pDate.getSeconds();
    else        
        this.Seconds=pDate.getSeconds();
        
    this.MyWindow=winCal;
    this.Ctrl=pCtrl;
    this.Format="ddMMyyyy";
    this.Separator=DateSeparator;
    this.ShowTime=false;
    if (pDate.getHours()<12)
        this.AMorPM="AM";
    else
        this.AMorPM="PM";    
}

function GetMonthIndex(shortMonthName)
{
    for (i=0;i<12;i++)
    {
        if (MonthName[i].substring(0,3).toUpperCase()==shortMonthName.toUpperCase())
        {    return i;}
    }
}
Calendar.prototype.GetMonthIndex=GetMonthIndex;

function IncYear()
{    Cal.Year++;}
Calendar.prototype.IncYear=IncYear;

function DecYear()
{    Cal.Year--;}
Calendar.prototype.DecYear=DecYear;
    
function SwitchMth(intMth)
{    Cal.Month=intMth;}
Calendar.prototype.SwitchMth=SwitchMth;

function SetHour(intHour)
{    
    var MaxHour;
    var MinHour;
    if (TimeMode==24)
    {    MaxHour=23;MinHour=0}
    else if (TimeMode==12)
    {    MaxHour=12;MinHour=1}
    else
        alert("TimeMode can only be 12 or 24");        
    var HourExp=new RegExp("^\\d\\d$");
    if (HourExp.test(intHour) && (parseInt(intHour,10)<=MaxHour) && (parseInt(intHour,10)>=MinHour))
    {    
        if ((TimeMode==12) && (Cal.AMorPM=="PM"))
        {
            if (parseInt(intHour,10)==12)
                Cal.Hours=12;
            else    
                Cal.Hours=parseInt(intHour,10)+12;
        }    
        else if ((TimeMode==12) && (Cal.AMorPM=="AM"))
        {
            if (intHour==12)
                intHour-=12;
            Cal.Hours=parseInt(intHour,10);
        }
        else if (TimeMode==24)
            Cal.Hours=parseInt(intHour,10);    
    }
}
Calendar.prototype.SetHour=SetHour;

function SetMinute(intMin)
{
    var MinExp=new RegExp("^\\d\\d$");
    if (MinExp.test(intMin) && (intMin<60))
        Cal.Minutes=intMin;
}
Calendar.prototype.SetMinute=SetMinute;

function SetSecond(intSec)
{    
    var SecExp=new RegExp("^\\d\\d$");
    if (SecExp.test(intSec) && (intSec<60))
        Cal.Seconds=intSec;
}
Calendar.prototype.SetSecond=SetSecond;

function SetAmPm(pvalue)
{
    this.AMorPM=pvalue;
    if (pvalue=="PM")
    {
        this.Hours=(parseInt(this.Hours,10))+12;
        if (this.Hours==24)
            this.Hours=12;
    }    
    else if (pvalue=="AM")
        this.Hours-=12;    
}
Calendar.prototype.SetAmPm=SetAmPm;

function getShowHour()
{
    var finalHour;
    if (TimeMode==12)
    {
        if (parseInt(this.Hours,10)==0)
        {
            this.AMorPM="AM";
            finalHour=parseInt(this.Hours,10)+12;    
        }
        else if (parseInt(this.Hours,10)==12)
        {
            this.AMorPM="PM";
            finalHour=12;
        }        
        else if (this.Hours>12)
        {
            this.AMorPM="PM";
            if ((this.Hours-12)<10)
                finalHour="0"+((parseInt(this.Hours,10))-12);
            else
                finalHour=parseInt(this.Hours,10)-12;    
        }
        else
        {
            this.AMorPM="AM";
            if (this.Hours<10)
                finalHour="0"+parseInt(this.Hours,10);
            else
                finalHour=this.Hours;    
        }
    }
    else if (TimeMode==24)
    {
        if (this.Hours<10)
            finalHour="0"+parseInt(this.Hours,10);
        else    
            finalHour=this.Hours;
    }    
    return finalHour;    
}                
Calendar.prototype.getShowHour=getShowHour;        

function GetMonthName(IsLong)
{
    var Month=MonthName[this.Month];
    if (IsLong)
        return Month;
    else
        return Month.substr(0,3);
}
Calendar.prototype.GetMonthName=GetMonthName;

function GetMonDays()//Get number of days in a month
{
    var DaysInMonth=[31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];
    if (this.IsLeapYear())
    {
        DaysInMonth[1]=29;
    }    
    return DaysInMonth[this.Month];    
}
Calendar.prototype.GetMonDays=GetMonDays;

function IsLeapYear()
{
    if ((this.Year%4)==0)
    {
        if ((this.Year%100==0) && (this.Year%400)!=0)
        {
            return false;
        }
        else
        {
            return true;
        }
    }
    else
    {
        return false;
    }
}
Calendar.prototype.IsLeapYear=IsLeapYear;

function FormatDate(pDate)
{
    if (this.Format.toUpperCase()=="DDMMYYYY")
        return (pDate+DateSeparator+(this.Month+1)+DateSeparator+this.Year);
    else if (this.Format.toUpperCase()=="DDMMMYYYY")
        return (pDate+DateSeparator+this.GetMonthName(false)+DateSeparator+this.Year);
    else if (this.Format.toUpperCase()=="MMDDYYYY")
        return ((this.Month+1)+DateSeparator+pDate+DateSeparator+this.Year);
    else if (this.Format.toUpperCase()=="MMMDDYYYY")
        return (this.GetMonthName(false)+DateSeparator+pDate+DateSeparator+this.Year);            
}
Calendar.prototype.FormatDate=FormatDate;

<html>
<head>
&lt;script src="datepicker.js"></script>
</head>
<body>
Pick a Date:
<form>
<input id="date1" type="text" name="date1" size="8" maxlength="10">
<a href="java script:NewCal('date1','mmddyyyy',false)"><img src="cal.gif" width="16" height="16" border="0" alt="Pick a date"></a>
</form>
</body>
</html>