How to fix jquery datepicker date calculation function using a drop down menu?
===

I am trying to create a script where depending on the value of a drop down box, it automaticly fills a textbox with a quarter, 2 quarters or 2 quarters away from the given date. So for value quarter 1, box1 would be "1-1-2011" and box2 would have "1-4-2011" filled in. At the moment I see nothing happening when it comes to changing dates. I am a beginner and have been trying to make this work for days.  Sorry if I've overlooked an obvious problem.

**The elements are as follows:**

    <div>
    <label>Periode</label>
    <select name="pchoice" id="pchoice"  onChange="calcdates()">
    <option value="moddate">Zelf invoeren</option>
    <option value="quarter1">1e Kwartaal</option>
    <option value="quarter2">2e Kwartaal</option>
    <option value="quarter3">3e Kwartaal</option>
    </select>
    </div> 
	<div> 
		<label>Datum </label> <input id="startDate" name="startDate" maxlength="10" style="width: 6em;"/> t/m <input id="endDate" name="endDate" maxlength="10" style="width: 6em;"/>
	</div>
   
**The script:**

    <script type="text/javascript" charset="utf-8">
	$(document).ready(
	function() {
	    var datepickerparam = {
	    dateFormat : 'dd-mm-yy',
            dayNamesMin : [ 'Zo', 'Ma', 'Di', 'Wo', 'Do', 'Vr', 'Za' ],
	    monthNames : [ 'Januari', 'Februari', 'Maart', 'April','Mei', 'Juni','Juli', 'Augustus', 'September','Oktober', 'November', 'December' ]
		};
							
	    $("#startDate").datepicker(datepickerparam);
	    $("#endDate").datepicker(datepickerparam);
	});
	
	function calcdates(){
   
	var w = document.pchoice.selectedIndex;
	var selected_text = document.pchoice.options[w].value;
	var commencedate =  new Date("01-01-2011"); (<%=UIHelper.toString(user.getContact().getOrganisation().getCommencingDate())%>);
	var end = new Date();
	//make dynamic with some variable
		switch(selected_text){
			case "quarter1":
				$("#startDate").val(commencedate);
				$("#endDate").val(commencedate+90);
            break;
			case "quarter2":
				$("#startDate").value='12-03-2011';
				$("#endDate").value='12-06-2011';
			break;
            case "quarter3":
				$("#startDate").value='13-06-2011';
				$("#endDate").value='13-09-2011';
			break;
            case "moddate":
				$("#startDate").value = '';
				$("#endDate").value ='';			  
			break;	
		}
	}
	    
</script>