zet dit in je head:

<script language="JavaScript1.2" type="text/javascript">
<!--
ns4 = (navigator.appName.indexOf("Netscape")>=0 && document.layers)? true : false;
ie4 = (document.all && !document.getElementById)? true : false;
ie5 = (document.all && document.getElementById)? true : false;
ns6 = (document.getElementById && navigator.appName.indexOf("Netscape")>=0 )? true: false;
w3c = (document.getElementById)? true : false;


var HTMLCode = "";
var DaysList = new Array("","Ma","Di","Wo","Do","Vr","Za","Zo");
var MonthsList = new Array("Mois_Vide","Januari","Februari","Maart","April","Mei","Juni","Juli","Augustus","September","Oktober","November","December");
var MonthLength = new Array("Mois_longueur_vide",31,29,31,30,31,30,31,31,30,31,30,31);

var QueryDate = 0;      
var QueryMonth = 0;     
var QueryYear = 0;      
var QueryDay = 0;       
var FirstDay = 0;       
var WeekRef = 0;        
var WeekOne = 0;        

var Today = new Date();
var TodaysYear = Today.getYear();
var TodaysMonth = Today.getMonth() + 1;
var TodaysDate = Today.getDate();
var TodaysDay = Today.getDay() + 1;
    if (TodaysYear < 2000) { TodaysYear += 1900; }

function CheckData()
{
QueryDate = document.Cal.Date.selectedIndex + 1;
QueryMonth = document.Cal.Month.selectedIndex + 1;
QueryYear = (document.Cal.Century.selectedIndex + 15) * 100 + document.Cal.Year.selectedIndex;
MonthLength[2] = CheckLeap(QueryYear);

if ((QueryYear * 10000 + QueryMonth * 100 + QueryDate) < 15821220)
    {
    alert("Je hebt een datum gekozen van voor Dec 20th 1582.\n\nKies een andere selectie.. zo ver terug gaan we niet !.");
    document.Cal.reset();
    CheckData();
    }
else if (MonthLength[QueryMonth] < QueryDate)       
    {
    alert("Il n'y a pas " + QueryDate + " jours en " + MonthsList[QueryMonth] + " " + QueryYear + " mais " + MonthLength[QueryMonth] + ". \nVeuillez choisir une autre date.");
    document.Cal.reset();
    CheckData();
    }
else { DisplaySchedule(); }

}


function CheckLeap(yy)
{
if ((yy % 100 != 0 && yy % 4 == 0) || (yy % 400 == 0)) { return 29; }
else { return 28; }
}


function DefWeekNum(dd)
{
numd = 0;
numw = 0;
for (n=1; n<QueryMonth; n++)
    {
    numd += MonthLength[n];
    }
numd = numd + dd - (9 - DefDateDay(QueryYear,1,1));
numw = Math.floor(numd / 7) + 1;

if (DefDateDay(QueryYear,1,1) == 1) { numw++; }
return numw;
}


function DefDateDay(yy,mm,dd)
{
return Math.floor((Date2Days(yy,mm,dd)-2) % 7) + 1;
}


function Date2Days(yy,mm,dd)
{
if (mm > 2)
    {
    var bis = Math.floor(yy/4) - Math.floor(yy/100) + Math.floor(yy/400);
    var zy = Math.floor(yy * 365 + bis);
    var zm = (mm-1) * 31 - Math.floor(mm * 0.4 + 2.3);
    return (zy + zm + dd);
    }
else
    {
    var bis = Math.floor((yy-1)/4) - Math.floor((yy-1)/100) + Math.floor((yy-1)/400);
    var zy = Math.floor(yy * 365 + bis);
    return (zy + (mm-1) * 31 + dd);
    }
}



function findid(name,doc){
var i,layer;
for(i=0;i<doc.layers.length;i++){
layer=doc.layers[i];
if(layer.name==name)return layer;
if(layer.document.layers.length>0)if((layer=findid(name,layer.document))!=null)return layer;
}
return null;
}




function DisplaySchedule()
{
HTMLCode = "<table cellspacing=0 cellpadding=3 border=1 bordercolor=#404040>";
QueryDay = DefDateDay(QueryYear,QueryMonth,QueryDate);
WeekRef = DefWeekNum(QueryDate);
WeekOne = DefWeekNum(1);
HTMLCode += "<tr align=center><td colspan=8 class=TITRE><b>" + MonthsList[QueryMonth] + " " + QueryYear + "</b></td></tr><tr align=center>";

for (s=1; s<8; s++)
    {
    if (QueryDay == s) { HTMLCode += "<td width=28><b><font color=#FF0000>" + DaysList[s] + "</font></b></td>"; }
    else { HTMLCode += "<td width=28><b>" + DaysList[s] + "</b></td>"; }
    }

HTMLCode += "<td><b><font color=#FF0000>Week</font></b></td></tr>";
a = 0;

for (i=(1-DefDateDay(QueryYear,QueryMonth,1)); i<MonthLength[QueryMonth]; i++)
    {
    HTMLCode += "<tr align=center>";
    for (j=1; j<8; j++)
        {
        if ((i+j) <= 0) { HTMLCode += "<td>&nbsp;</td>"; }
        else if ((i+j) == QueryDate) { HTMLCode += "<td><b><font color=#FF9933>" + (i+j) + "</font></b></td>"; }
        else if ((i+j) > MonthLength[QueryMonth]) { HTMLCode += "<td>&nbsp;</td>"; }
        else { HTMLCode += "<td>" + (i+j) + "</td>"; }
        }

    if ((WeekOne+a) == WeekRef) { HTMLCode += "<td><b><font color=#FF0000>" + WeekRef + "</font></b></td>"; }
    else { HTMLCode += "<td><font color=#99CCCC>" + (WeekOne+a) + "</font></td>"; }
    HTMLCode += "</tr>";
    a++;
    i = i + 6;
    }

// ALLOWS CROSS-BROWSER WRITING IN DOCUMENT. BEGIN BG MODIFY
if(ie4)document.all['Calendrier'].innerHTML = HTMLCode + "</table>";
if(w3c)document.getElementById('Calendrier').innerHTML= HTMLCode + "</table>";
if(ns4){
var tns4=findid('Calendrier',document);
tns4.document.open();
tns4.document.write('<center>'+HTMLCode+'</table></center>');
tns4.document.close();
}
// END BG MODIFY

}

function Back()
{
document.Cal.Month.selectedIndex = document.Cal.Month.selectedIndex - 1;
if (document.Cal.Month.selectedIndex == -1) { document.Cal.Month.selectedIndex = 11; }
}

function Next()
{
document.Cal.Month.selectedIndex = document.Cal.Month.selectedIndex + 1;
if (document.Cal.Month.selectedIndex == -1) { document.Cal.Month.selectedIndex = 0; }
}


window.onload=CheckData;
if(ns4)window.onresize=function(){
	setTimeout('history.go(0)',400);
	}

//-->
</script>						


zet dit in je body



<center>


<form name="Cal">
<font color="#00FF00">
<script language="JavaScript1.2" type="text/javascript">
    <!--
 
    DateText = "<select name=\"Date\">"
    for (d=1; d<32; d++)
        {
        DateText += "<option";
        if (d == TodaysDate) { DateText += " SELECTED"; }
        DateText += ">";
        if (d < 10) { DateText += "0"; }
        DateText += d + "</option>";
        }
    DateText += "</select>";
    //*************************************************
    MonthText = "<select name=\"Month\">"
    for (m=1; m<13; m++)
        {
        MonthText += "<option";
        if (m == TodaysMonth) { MonthText += " SELECTED"; }
        MonthText += ">";
        MonthText += MonthsList[m] + "</option>";
        }
    MonthText += "</select>";
    //*************************************************
    CenturyText = "<select name=\"Century\">"
    for (c=15; c<25; c++)
        {
        CenturyText += "<option";
        if (c == Math.floor(TodaysYear / 100)) { CenturyText += " SELECTED"; }
        CenturyText += ">" + c + "</option>";
        }
    CenturyText += "</select>";
    //*************************************************
    YearText = "<select name=\"Year\">";
    for (y=0; y<100; y++)
        {
        YearText += "<option";
        if (y == (TodaysYear - Math.floor(TodaysYear / 100) * 100)) { YearText += " SELECTED"; }
        YearText += ">";
        if (y < 10) { YearText += "0"; }
        YearText += y + "</option>";
        }
    YearText += "</select>";
    //*************************************************
    document.write(MonthText+' '+DateText+', '+CenturyText+' '+YearText);
    //-->
</script>  <input type="button" value=" OK " style="font-size: 8 pt" onClick="CheckData()"><br><br>
<input type="button" value="<< Maand" style="font-size: 8 pt" onClick="Back(); CheckData()"><input type="button" value=" Ga terug naar vandaag..." style="font-size: 8 pt" onClick="document.Cal.reset(); setTimeout('CheckData()',100)"><input type="button" value="Maand >>" style="font-size: 8 pt" onClick="Next();CheckData()">
<br><br><br>
<script language="javascript1.2">
if(ns4)document.write('<layer name="Calendrier"></layer><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br><br>')
else document.write('<div id="Calendrier"></div>');
</script>
</font>
</center></form>